MB-4334 - clear downstream timeout before releasing
MB-4334 - clear downstream timeout before releasing

There's one code path in cproxy_release_downstream() where a
cproxy_forward() fails, during a retry, possibly while the downstream
timeout_tv event is still regsitered.  In that case, a downstream
could be released that has a non-0 timeout_tv.  It's a small window
that some users have hit.

Change-Id: I1b87298dba1151c8ece51c0cd78d68ca6fa2bdb0
Reviewed-on: http://review.couchbase.org/15232
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

zbase/moxi,zbase/moxi,zbase/moxi,zbase/moxi,zbase/moxi