MB-19897: Only update sendTime if successfully send noop

MB-19897: Only update sendTime if successfully send noop

In the maybeSendNoop function when a DCP producer attempts
to send a noop to a consumer it can receive back
ENGINE_SUCCESS or ENGINE_E2BIG.

We should only set pendingRecv to true and update the
last sendTime if ENGINE_SUCCESS is returned.

Change-Id: Ice8a66dcae35505d7bab7d261f080d5ffb95c8e3
Reviewed-on: http://review.couchbase.org/64878
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>