Remove dead code

- Fixed #1557
Fix embarassing compilation error :-/
Add a boolean parameter to Channel.Unsafe.flush() so that SelectionKey state check can be bypassed.
Make the cumulation field in ByteToMessageDecoder package private to hide it
|| -> &&
Remove UnreplayableOperationException

- Can't find a reason to keep it.
Kill the cruft
Add javadocs to methods that user may override and make the others final
Update dependency versions
Some javadocs
[#1557] Make the contract of Channel.Unsafe.flush() more clear

* Remove boolean parameter from Channel.Unsafe.flush() method
* Move NIO related things to AbstractNioChannel.NioUnsafe
[#1573] Fix NPE which could be triggered one failed promises that trigger a close89
Hide RecyclableArrayList from sub-classes
Add and correct javadocs
[1575] Correctly parse Content-Type value
Just some tiny javadocs optimizations
Remove unused class which was package private anyway
More javadoc fixes
Fix checkstyle
Fix checkstyle