Synchronized between 4.1 and master again (part 2)

Motivation:
4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:
Remove ChannelHandlerInvoker.writeAndFlush(...) and the related
implementations.

Result:
4.1 and master got closer.