netty: Handle shutdown and failures during negotiator

NettyClientTransport needs to call close() on the Channel directly
instead of sending a message, since the message would typically be
delayed until negotiation completes.

The closeFuture() closes too early to be helpful, which is very
unfortunate. Using it squelches the negotiator's error handling. We now
rely on the handlers to report shutdown without any back-up. The
handlers error handling has matured, so maybe this is okay.