Fix resource leakage in example
Add missing javadocs
Introduce ChannelGroupMatcher which can be used to only apply operations of a ChannelGroup on matching Channels.

This is often useful if you for example use a ChannelGroup to hold all connected Channels and want to broadcast a message too all of them
except one Channel.
[#1561] Fix Securechat example which was broken because Channel was not flushed
Merge remote-tracking branch upstream/$:{upbranch}