SPDY: refactor frame codec implementation

Motivation:

Currently, the SPDY frame encoding and decoding code is based upon
the ChannelHandler abstraction. This requires maintaining multiple
versions for 3.x and 4.x (and possibly 5.x moving forward).

Modifications:

The SPDY frame encoding and decoding code is separated from the
ChannelHandler and SpdyFrame abstractions. Also test coverage is
improved.

Result:

SpdyFrameCodec now implements the ChannelHandler abstraction and is
responsible for creating and handling SpdyFrame objects.
Add missing override annotations
Cleaning up code for determining whether the HTTP/2 connection header
and initial settings frame were sent.

Motivation:

The current logic for marking the connection header and initial settings
frame is a bit complicated.

Modifications:

- Http2FrameEncoder, Http2ConnectionHandler: removed sent listeners and
set the sent flag immediately.

Result:

Just a code cleanup ... behavior is the same.
HTTP2 example server should also log.

Motivation:

The HTTP2 example client logs, and it's useful to show what's
going on.  It'd be sweet if the server did too.

Modifications:

Added Http2FrameLogger to example server pipeline.

Result:

HTTP2 example server will log frames.
Fix buffer leaks in SPDY test

Motivation:

Fix leaks reported during SPDY test.

Modifications:

Use ReferenceCountUtil.releaseLater(...) to make sure everything is released once the tests are done.

Result:

No more leak reports during run the tests.
Fix buffer leaks in SpdyFrameDecoderTest

Motivation:

Fix leaks reported during running SpdyFrameDecoderTest

Modifications:
Make sure the produced buffer of SpdyFrameDecoder is released

Result:

No more leak reports during run the tests.
 Fix missed buffer leaks in SpdyFrameDecoderTest

 Motivation:
 Fix leaks reported during running SpdyFrameDecoderTest

Modifications:
Make sure the produced buffers of SpdyFrameDecoder and SpdyFrameDecoderTest are released

Result:

No more leak reports during run the tests.
[#2376] Add support for SO_REUSEPORT in native transport

Motivation:
In linux kernel 3.9 a new featured named SO_REUSEPORT was introduced which allows to have multiple sockets bind to the same port and so handle the accept() of new connections with multiple threads. This can greatly improve the performance when you not to accept a lot of connections.

Modifications:
Implement SO_REUSEPORT via JNI

Result:
Be able to use the SO_REUSEPORT feature when using the EpollServerSocketChannel
[#2377] Implement epoll based DatagramChannel

Motivation:
There is currently no epoll based DatagramChannel. We should add one to make the set of provided channels complete and also to be able to offer better performance compared to the NioDatagramChannel once SO_REUSEPORT is implemented.

Modifications:
Add implementation of DatagramChannel which uses epoll. This implementation does currently not support multicast yet which will me implemented later on. As most users will not use multicast anyway I think it is fair to just add the EpollDatagramChannel without the support for now. We shipped NioDatagramChannel without support earlier too ...

Result:
Be able to use EpollDatagramChannel for max. performance on linux
[#2375] [#2404] Fix bug in respecting ChannelConfig.setAutoRead(false) and also fix Channel.read() for OIO

Motivation:
At the moment ChanneConfig.setAutoRead(false) only is guaranteer to not have an extra channelRead(...) triggered when used from within the channelRead(...) or channelReadComplete(...) method. This is not the correct behaviour as it should also work from other methods that are triggered from within the EventLoop. For example a valid use case is to have it called from within a ChannelFutureListener, which currently not work as expected.

Beside this there is another bug which is kind of related. Currently Channel.read() will not work as expected for OIO as we will stop try to read even if nothing could be read there after one read operation on the socket (when the SO_TIMEOUT kicks in).

Modifications:
Implement the logic the right way for the NIO/OIO/SCTP and native transport, specific to the transport implementation. Also correctly handle Channel.read() for OIO transport by trigger a new read if SO_TIMEOUT was catched.

Result:
It is now also possible to use ChannelConfig.setAutoRead(false) from other methods that are called from within the EventLoop and have direct effect.
[#2405] Add support for SO_REUSEPORT to EpollDatagramChannel

Motivation:
With SO_REUSEPORT it is possible to bind multiple sockets to the same port and so handle the processing of packets via multiple threads. This allows to handle DatagramPackets with more then one thread on the same port and so gives better performance.

Modifications:
Expose EpollDatagramChannelConfig.setReusePort(..) and isReusePort()

Result:
Allow to bind multiple times to the same local address and so archive better performance.
[#2144] Fix NPE in Local transport caused by a race

Motivation:
At the moment it is possible to see a NPE when the LocalSocketChannels doRegister() method is called and the LocalSocketChannels doClose() method is called before the registration was completed.

Modifications:
Make sure we delay the actual close until the registration task was executed.

Result:
No more NPE
[#2401] Improve documentation of HttpObjectAggregator

Motivation:
Make it more clear what the output of HttpObjectAggregator is and that it need to come after the encoder in the pipeline.

Modifications:
Change javadocs to make things more clear.

Result:
Better docs
Move validatePromise(...) to ChannelHandlerInvokerUtil. Related to [#2398]

Motivation:
Once a user implement a custom ChannelHandlerInvoker it is needed to validate the ChannelPromise. We should expose a utility method for this.

Modifications:
Move validatePromise(...) from DefaultChannelHandlerInvoker to ChannelHandlerInvokerUtil and make it public.

Result:
User is able to reuse code
HTTP2 example uses SSL and NPN.

Motivation:

HTTP2 is generally negotiated over SSL, makes more sense to provide an SSL example.

Modifications:

Copy the SDPY example to add SSL and NPN negotiation to the HTTP2 example.

Result:

Http2Server and Http2Client examples use SSL.
HTTP2 server should not send preface string

Motivation:

See: https://github.com/netty/netty/issues/2402
See: https://tools.ietf.org/html/draft-ietf-httpbis-http2-10#section-3.5

Only the client should send the preface string, the server should not.

"The server connection header consists of just a SETTINGS frame (Section 6.5)
 that MUST be the first frame the server sends in the HTTP/2 connection."

Modifications:

Split out Http2ClientPrefaceWriter and Http2ServerPrefaceReader from
Http2FrameEncoder and Http2FrameDecoder.  The new channel handlers are added
to the pipeline and remove themselves after the preface is written/read.

Result:

HTTP2 client sends preface, server does not, in compliance with spec.
Work around an Android SSLEngine issue

Motivation:

Some Android SSLEngine implementations skip FINISHED handshake status
and go straightly into NOT_HANDSHAKING.  This behavior blocks SslHandler
from notifying its handshakeFuture, because we do the notification when
SSLEngine enters the FINISHED state.

Modification:

When the current handshake state is NOT_HANDSHAKING and the
handshakeFuture is not fulfilled yet, treat NOT_HANDSHAKING as FINISHED.

Result:

Better Android compatibility - fixes #1823
[#2396] Allow to set TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT in native transport

Motivation:
Allow to set TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT in native transport to offer the user with more flexibility.

Modifications:
Expose methods to set these options and write the JNI implementation.

Result:
User can now use TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT.
Fix buffer leak in EpollDatagramChannel

Motivation:
EpollDatagramChannel produced buffer leaks when tried to read from the channel and nothing was ready to be read.

Modifications:
Correctly release buffer if nothing was read

Result:
No buffer leak
Fix chunk type for stream identifier

Motivation:
The problem with the current snappy implementation is that it does
not comply with framing format definition found on
https://code.google.com/p/snappy/source/browse/trunk/framing_format.txt

The document describes that chunk type of the stream identifier is defined
as 0xff. The current implentation uses 0x80.

Modifications:
This patch replaces the first byte of the chunk type of the stream identifier
with 0xff.

Result:
After this modification the snappy implementation is compliant to the
framing format described at
https://code.google.com/p/snappy/source/browse/trunk/framing_format.txt.
This results in a better compatibility with other implementations.