Remove unnecessary try-catch block in AbstractUnsafe.flush()
Fix a bug where ChannelOutboundBuffer.addFlush() is called multiple times
Reduce garbage in AbstractUnsafe.flush0()
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.
Merge remote-tracking branch upstream/$:{upbranch}