Netty Headers Class Restructure and Algorithm Updates

Motivation:
Headers within netty do not cleanly share a common class hierarchy.  As a result some header types support some operations
and don't support others.  The consolidation of the class hierarchy will allow for maintenance and scalability for new codec.
The existing hierarchy also has a few short comings such as it is not clear when data conversions are happening.  This
could result unintentionally getting back a collection or iterator where a conversion on each entry must happen.

The current headers algorithm also prepends all elements which means to find the first element or return a collection
in insertion order often requires a complete traversal followed by a collections.reverse call.

Modifications:
-Provide a generic base class which provides all the implementation for headers in netty
-Provide an extension to this class which allows for name type conversions to happen (to accommodate legacy CharSequence to String conversions)
-Update the headers interface to clarify when conversions will happen.
-Update the headers data structure so that appends are done to avoid unnecessary iteration or collection reversal.

Result:
-More unified class hierarchy for headers in netty
-Improved headers data structure and algorithms
-headers API more clearly identify when conversions are required.
CorsHandler should release HttpRequest after processing preflight/error.

Motivation:
Currently, when the CorsHandler processes a preflight request, or
respondes with an 403 Forbidden using the short-curcuit option, the
HttpRequest is not released which leads to a buffer leak.

Modifications:
Releasing the HttpRequest when done processing a preflight request or
responding with an 403.

Result:
Using the CorsHandler will not cause buffer leaks.
Handle an empty ByteBuf specially in HttpObjectEncoder

Related: #2983

Motivation:

It is a well known idiom to write an empty buffer and add a listener to
its future to close a channel when the last byte has been written out:

  ChannelFuture f = channel.writeAndFlush(Unpooled.EMPTY_BUFFER);
  f.addListener(ChannelFutureListener.CLOSE);

When HttpObjectEncoder is in the pipeline, this still works, but it
silently raises an IllegalStateException, because HttpObjectEncoder does
not allow writing a ByteBuf when it is expecting an HttpMessage.

Modifications:

- Handle an empty ByteBuf specially in HttpObjectEncoder, so that
  writing an empty buffer does not fail even if the pipeline contains an
  HttpObjectEncoder
- Add a test

Result:

An exception is not triggered anymore by HttpObjectEncoder, when a user
attempts to write an empty buffer.
Overall clean-up on Headers and its subtypes

Motivation:

- There are still various inspector warnings to fix.
- ValueConverter.convert() methods need to end with the type name like
  other methods in Headers, such as setInt() and addInt(), for more
  consistency

Modifications:

- Fix all inspector warnings
- Rename ValueConverter.convert() to convert<type>()

Result:

- Cleaner code
- Consistency
Headers Cleanup

Motiviation:
Some of the contains method signatures did not include the correct type of parameters for the method names.  Also these same methods were not using the correct conversion methods but instead the generic object conversion methods.

Modifications
-Correct the parameter types of the contains methods to match the method name
-Correct the implementation of these methods to use the correct conversion methods

Result:
Headers contains signatures are more correct and correct conversion methods are used.
Don't close the connection whenever Expect: 100-continue is missing.

Motivation:

The 4.1.0-Beta3 implementation of HttpObjectAggregator.handleOversizedMessage closes the
connection if the client sent oversized chunked data with no Expect:
100-continue header. This causes a broken pipe or "connection reset by
peer" error in some clients (tested on Firefox 31 OS X 10.9.5,
async-http-client 1.8.14).

This part of the HTTP 1.1 spec (below) seems to say that in this scenario the connection
should not be closed (unless the intention is to be very strict about
how data should be sent).

http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html

"If an origin server receives a request that does not include an
Expect request-header field with the "100-continue" expectation,
the request includes a request body, and the server responds
with a final status code before reading the entire request body
from the transport connection, then the server SHOULD NOT close
the transport connection until it has read the entire request,
or until the client closes the connection. Otherwise, the client
might not reliably receive the response message. However, this
requirement is not be construed as preventing a server from
defending itself against denial-of-service attacks, or from
badly broken client implementations."

Modifications:

Change HttpObjectAggregator.handleOversizedMessage to close the
connection only if keep-alive is off and Expect: 100-continue is
missing. Update test to reflect the change.

Result:

Broken pipe and connection reset errors on the client are avoided when
oversized data is sent.
Implement user-defined writability flags

Related: #2945

Motivation:

Some special handlers such as TrafficShapingHandler need to override the
writability of a Channel to throttle the outbound traffic.

Modifications:

Add a new indexed property called 'user-defined writability flag' to
ChannelOutboundBuffer so that a handler can override the writability of
a Channel easily.

Result:

A handler can override the writability of a Channel using an unsafe API.
For example:

  Channel ch = ...;
  ch.unsafe().outboundBuffer().setUserDefinedWritability(1, false);
Fix the websocket server example

Motivation:
As report in #2953 the websocket server example contained a bug and did therefore not work with chrome:
A websocket extension is added to the pipeline but extensions were disallowed in the handshaker and decoder,
which is leading the decoder to closing the connection after receiving an extension frame.

Modifications:
Allow websocket extensions in the handshaker to correctly enable the extension.

Result:
Working websocket server example
Fixes #2953
Modify HttpObjectDecoder to allow parsing the HTTP headers in multiple steps.

Motivation:
At the moment the whole HTTP header must be parsed at once which can lead to multiple parsing of the same bytes. We can do better here and allow to parse it in multiple steps.

Modifications:

 - Not parse headers multiple times
 - Simplify the code
 - Eliminate uncessary String[] creations
 - Use readSlice(...).retain() when possible.

Result:

Performance improvements as shown in the included benchmark below.

Before change:
[nmaurer@xxx]~% ./wrk-benchmark
Running 2m test @ http://xxx:8080/plaintext
  16 threads and 256 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    21.55ms   15.10ms 245.02ms   90.26%
    Req/Sec   196.33k    30.17k  297.29k    76.03%
  373954750 requests in 2.00m, 50.15GB read
Requests/sec: 3116466.08
Transfer/sec:    427.98MB

After change:
[nmaurer@xxx]~% ./wrk-benchmark
Running 2m test @ http://xxx:8080/plaintext
  16 threads and 256 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    20.91ms   36.79ms   1.26s    98.24%
    Req/Sec   206.67k    21.69k  243.62k    94.96%
  393071191 requests in 2.00m, 52.71GB read
Requests/sec: 3275971.50
Transfer/sec:    449.89MB
Overall cleanup of 2b191fe6e90c8d99ffa221a3fa523d6bc8de895a
Fix a compilation error
Improve DnsNameResolverTest.testResolveA()

Motivation:

DnsNameResolver.testResolveA() tests if the cache works as well as the usual DNS protocol test.  To ensure the result from the cache is identical to the result without cache, it compares the two Maps which contain the result of cached/uncached resolution.  The comparison of two Maps yields an expected behavior, but the output of the comparison on failure is often unreadable due to its long length.

Modifications:

Compare entry-by-entry for more comprehensible test failure output

Result:

When failure occurs, it's easier to see which domain was the cause of the problem.
Added an option to use websockets without masking

Motivation:

The requirement for the masking of frames and for checks of correct
masking in the websocket specifiation have a large impact on performance.
While it is mandatory for browsers to use masking there are other
applications (like IPC protocols) that want to user websocket framing and proxy-traversing
characteristics without the overhead of masking. The websocket standard
also mentions that the requirement for mask verification on server side
might be dropped in future.

Modifications:

Added an optional parameter allowMaskMismatch for the websocket decoder
that allows a server to also accept unmasked frames (and clients to accept
masked frames).
Allowed to set this option through the websocket handshaker
constructors as well as the websocket client and server handlers.
The public API for existing components doesn't change, it will be
forwarded to functions which implicetly set masking as required in the
specification.
For websocket clients an additional parameter is added that allows to
disable the masking of frames that are sent by the client.

Result:

This update gives netty users the ability to create and use completely
unmasked websocket connections in addition to the normal masked channels
that the standard describes.
Minor update to Http2LifecycleManager#closeLocalSide javadoc

Modifications:
Changed "remote" to "local" in javadoc for closeLocalSide.
HTTP/2 Send GOAWAY if no more stream ids

Motivation:
If the http2 encoder has exhausted all available stream IDs a GOAWAY frame is not sent. Once the encoder detects the a new stream ID has rolled over past the last stream ID a GOAWAY should be sent as recommended in section [5.1.1](https://tools.ietf.org/html/draft-ietf-httpbis-http2-14#section-5.1.1).

Modifications:
-This condition is already detected but it just needs to result in a GOAWAY being sent.
-Add a subclass of Http2Exception so the encoder can detect this special case.
-Add a unit test which checks that the GOAWAY is sent/received.

Result:
Encoder attempting to use the first 'rolled over' stream id results in a GOAWAY being sent.
SslHandler wrap memory leak

Motivation:
The SslHandler wrap method requires that a direct buffer be passed to the SSLEngine.wrap() call. If the ByteBuf parameter does not have an underlying direct buffer then one is allocated in this method, but it is not released.

Modifications:
- Release the direct ByteBuffer only accessible in the scope of SslHandler.wrap

Result:
Memory leak in SslHandler.wrap is fixed.
SslHander wrap conditional direct buffer allocation

Motivation:
The SslHandler currently forces the use of a direct buffer for the input to the SSLEngine.wrap(..) operation. This allocation may not always be desired and should be conditionally done.

Modifications:
- Use the pre-existing wantsDirectBuffer variable as the condition to do the conversion.

Result:
- An allocation of a direct byte buffer and a copy of data is now not required for every SslHandler wrap operation.
ALPN should allow handshake failure if no compatible protocols found

Motivation:
If there are no common protocols in the ALPN protocol exchange we still compete the handshake successfully.  This handshake should fail according to http://tools.ietf.org/html/rfc7301#section-3.2 with a status of no_application_protocol.  The specification also allows for the server to "play dumb" and not advertise that it supports ALPN in this case (see MAY clauses in http://tools.ietf.org/html/rfc7301#section-3.1)

Modifications:
-Upstream project used for ALPN (alpn-boot) does not support this.  So a PR https://github.com/jetty-project/jetty-alpn/pull/3 was submitted.
-The netty code using alpn-boot should support the new interface (return null on existing method).
-Version number of alpn-boot must be updated in pom.xml files

Result:
-Netty fails the SSL handshake if ALPN is used and there are no common protocols.
SSL JDK Mutual Authentication

Motivation:
Netty currently does not support creating SslContext objects that support mutual authentication.

Modifications:
-Modify the SslContext interface to support mutual authentication for JDK and OpenSSL
-Provide an implementation of mutual authentication for JDK
-Add unit tests to support new feature

Result:
Netty SslContext interface supports mutual authentication and JDK providers have an implementation.
Code clean-up

- Fix the inspector warnings
- Fix the infinite recursion in SslContext.newClientContext()
- Fix Javadoc errors