Fix hang on new request receives after shutdown.

Previously, requests could still be sent to a shutdown client and
added to the client's internal data structures, without ever
responding to the future with an error for a shutdown client (causing
hangs when blocking on the future receive).  This change fixes this by
performing a non-blocking read of the client's shutdown channel before
adding a request, and responding with the shutdown error if the client
has begun or completed its shutdown.

ok @davecgh