Use ByteBuf.readSlice(...).retain() to minimize memory copies.

Motivation:
At the moment we call ByteBuf.readBytes(...) in these handlers but with optimizations done as part of 25e0d9d we can just use readSlice(...).retain() and eliminate the memory copy.

Modifications:
Replace ByteBuf.readBytes(...) usage with readSlice(...).retain().

Result:
Less memory copies.
Merging HTTP/2 preface logic into connection handler

Motivation:

The HTTP/2 connection preface logic is currently handled in two places.
Reading/writing the client preface string is handled by
Http2PrefaceHandler while the reading/writing of the initial settings
frame is handled by AbstractHttp2ConnectionHandler. Given that their
isn't much code in Http2PrefaceHandler, it makes sense to just merge it
into the preface handling logic of AbstractHttp2ConnectionHandler. This
will also make configuring the pipeline simpler for HTTP/2.

Modifications:

Removed Http2PrefaceHandler and added it's logic to
AbstractHttp2ConnectionHandler. Updated other classes depending on
Http2PrefaceHandler.

Result:

All of the HTTP/2 connection preface processing logic is now in one
place.
More robust native library discovery in Mac OS X

Motivation:

Some JDK versions of Mac OS X generates a JNI dynamic library with '.jnilib' extension rather than with '.dynlib' extension.  However, System.mapLibraryName() always returns 'lib<name>.dynlib'. As a result, NativeLibraryLoader fails to load the native library whose extension is .jnilib.

Modification:

Try to find both '.jnilib' and '.dynlib' resources on OS X.

Result:

Dynamic libraries are loaded correctly in Mac OS X, and thus we can continue the OpenSslEngine work.
Minimize memory footprint of HashedWheelTimer and context-switching

Motivation:
At the moment there are two issues with HashedWheelTimer:
* the memory footprint of it is pretty heavy (250kb fon an empty instance)
* the way how added Timeouts are handled is inefficient in terms of how locks etc are used and so a lot of context-switching / condition can happen.

Modification:
Rewrite HashedWheelTimer to use an optimized bucket implementation to store the submitted Timeouts and a MPSC queue to handover the timeouts.  So volatile writes are reduced to a minimum and also the memory foot-print of the buckets itself is reduced a lot as the bucket uses a double-linked-list. Beside this we use Atomic*FieldUpdater where-ever possible to improve the memory foot-print and performance.

Result:
Lower memory-footprint and better performance
Not throw a Http2Exception when a ChannelFuture is returned

Motivation:
At the moment we have some methods that return a ChannelFuture but still throw a Http2Exception too. This is confusing in terms of semantic. A method which returns a ChannelFuture should not throw an Http2Exception but just fail the ChannelFuture.

Modifications:
* Make sure we fail the returned ChannelFuture in cases of Http2Exception and remove the throws Http2Exception from the method signature.
* Also some cleanup

Result:
Make the API usage more clear.
Fix the life cycle mapping problem with m2e

Motivation:

It should be frictionless to import our project into Eclipse

Modifications:

Exclude the plugins with missing life cycle mapping. They are not useful
for use with IDE anyway.

Result:

Fixes #2488
Netty is imported into Eclipse without a problem.
Fix buffer leaks in HTTP2
Fix capacity check bug affecting offheap buffers
Correctly release buffer when testing DelegatingHttp2ConnectionHandler
Better implementation of AttributeMap and also add hasAttr(...). See [#2439]

Motivation:
The old DefaultAttributeMap impl did more synchronization then needed and also did not expose a efficient way to check if an attribute exists with a specific key.

Modifications:
* Rewrite DefaultAttributeMap to not use IdentityHashMap and synchronization on the map directly. The new impl uses a combination of AtomicReferenceArray and synchronization per chain (linked-list). Also access the first Attribute per bucket can be done without any synchronization at all and just uses atomic operations. This should fit for most use-cases pretty weel.
* Add hasAttr(...) implementation

Result:
It's now possible to check for the existence of a attribute without create one. Synchronization is per linked-list and the first entry can even be added via atomic operation.
Preparation for porting OpenSSL support in 3.10

- Add licenses and dependencies
Add an OpenSslEngine and the universal API for enabling SSL

Motivation:

Some users already use an SSLEngine implementation in finagle-native. It
wraps OpenSSL to get higher SSL performance.  However, to take advantage
of it, finagle-native must be compiled manually, and it means we cannot
pull it in as a dependency and thus we cannot test our SslHandler
against the OpenSSL-based SSLEngine.  For an instance, we had #2216.

Because the construction procedures of JDK SSLEngine and OpenSslEngine
are very different from each other, we also need to provide a universal
way to enable SSL in a Netty application.

Modifications:

- Pull netty-tcnative in as an optional dependency.
  http://netty.io/wiki/forked-tomcat-native.html
- Backport NativeLibraryLoader from 4.0
- Move OpenSSL-based SSLEngine implementation into our code base.
  - Copied from finagle-native; originally written by @jpinner et al.
  - Overall cleanup by @trustin.
- Run all SslHandler tests with both default SSLEngine and OpenSslEngine
- Add a unified API for creating an SSL context
  - SslContext allows you to create a new SSLEngine or a new SslHandler
    with your PKCS#8 key and X.509 certificate chain.
  - Add JdkSslContext and its subclasses
  - Add OpenSslServerContext
- Add ApplicationProtocolSelector to ensure the future support for NPN
  (NextProtoNego) and ALPN (Application Layer Protocol Negotiation) on
  the client-side.
- Add SimpleTrustManagerFactory to help a user write a
  TrustManagerFactory easily, which should be useful for those who need
  to write an alternative verification mechanism. For example, we can
  use it to implement an unsafe TrustManagerFactory that accepts
  self-signed certificates for testing purposes.
- Add InsecureTrustManagerFactory and FingerprintTrustManager for quick
  and dirty testing
- Add SelfSignedCertificate class which generates a self-signed X.509
  certificate very easily.
- Update all our examples to use SslContext.newClient/ServerContext()
- SslHandler now logs the chosen cipher suite when handshake is
  finished.

Result:

- Cleaner unified API for configuring an SSL client and an SSL server
  regardless of its internal implementation.
- When native libraries are available, OpenSSL-based SSLEngine
  implementation is selected automatically to take advantage of its
  performance benefit.
- Examples take advantage of this modification and thus are cleaner.
Optimize SslHandler in an OpenSslEngine-friendly way

Motivation:

Previous fix for the OpenSslEngine compatibility issue (#2216 and
18b0e95659c057b126653bad2f018a8ce5385255) was to feed SSL records one by
one to OpenSslEngine.unwrap().  It is not optimal because it will result
in more JNI calls.

Modifications:

- Do not feed SSL records one by one.
- Feed as many records as possible up to MAX_ENCRYPTED_PACKET_LENGTH
- Deduplicate MAX_ENCRYPTED_PACKET_LENGTH definitions

Result:

- No allocation of intemediary arrays
- Reduced number of calls to SSLEngine and thus its underlying JNI calls
- A tad bit increase in throughput, probably reverting the tiny drop
  caused by 18b0e95659c057b126653bad2f018a8ce5385255
Fix JDK 8 compatibility issue with OpenJdkSelfSignedCertGenerator

- X509CertInfo.setSubject/setIssuer() requires X500Name instead of
  CertificateSubjectName/CertificateIssuerName.
Fix buffer leaks during PEM to KeyStore conversion
Add netty-tcnative dependency to netty-example

Motivation:

It's useful to have netty-tcnative dependency in netty-example because
we can play with OpenSslEngine from our IDE.

Modifications:

Add netty-tcnative to example/pom.xml
Reduce memory usage of SslHandler when OpenSslEngine is in use

Motivation:

JDK's SSLEngine.wrap() requires the output buffer to be always as large as MAX_ENCRYPTED_PACKET_LENGTH even if the input buffer contains small number of bytes.  Our OpenSslEngine implementation does not have such wasteful behaviot.

Modifications:

If the current SSLEngine is OpenSslEngine, allocate as much as only needed.

Result:

Less peak memory usage.
Upgrade os-maven-plugin to fix the build issue
Add JVM crash logs to .gitignore
Update os-maven-plugin again to address an IDEA integration issue