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.