MB-11843 dcp_client: Send error response to waiters on dcp connection close

When tcp connection is closed by EP-Engine, the dcp client should send
an error reply to all the pending active requests.

Change-Id: I7b6f9fa26d651120519b5fa54f8d830a39e8c543
Reviewed-on: http://review.couchbase.org/40256
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>