Return want more when noop or flow control is successful

We want the memcached layer to call into our upr connection if there
is any sort of success on the previous call.

Change-Id: I092f92ef5a1de937b015a2f91e17c4306943d023
Reviewed-on: http://review.couchbase.org/39554
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>