Race condition with completion of Channel closeFuture and unregistration.

Motivation:
AbstractChannel.close() completes the closeFuture and then proceeds to
schedule a task to fireChannelInactive and unregister the channel. If
the user shuts down the EventLoop as a result of the closeFuture
completing this can result in a RejectedExecutionException when the
unregister task is scheduled.  This is mostly noise, but it does
somewhat pollute the logs.

Modifications:
Changed AbstractChannel.close() to not complete the channelFuture and
promise until after the unregistration task is scheduled.

Result:
Noisy error log should no longer be an issue.