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