Relax Maven version requirement
[#2079] Stop reading once the NIO byte Channel was complete drained
More graceful registration failure

- Fixes #2060
- Ensure to return a future/promise implementation that does not fail with 'not registered to an event loop' error for registration operations
- If there is no usable event loop available, GlobalEventExecutor.INSTANCE is used as a fallback.
- Add VoidChannel, which is used when an instantiation of a channel fails.
Fix a bug where adaptive recvbuf size prediction doesn't work correctly when maxMessagesPerRead is > 1
Increase the default maxMessagesPerRead of  AbstractNioByteChannel to 16

 - Related: #2079
Use LinkedHashSet for HttpHeaders.names() and add Javadoc about the performance of names() and entries()
Workaround for a bug in maven-release-plugin where it doesn't work well with Git 1.8.5
[maven-release-plugin] prepare release netty-5.0.0.Alpha1
[maven-release-plugin] prepare for next development iteration