MB-9715: Add UPR Consumer connection pausing

In the current code the upr thread continually calls the step
function for any consumer connection. This change allows an UPR
Consumer to notify the memcached layer when it does and	 does not
have data ready. It will fix the 100% cpu issue that occurs when
using an UPR Consumer.

Change-Id: I122faa8621b340dbc4e8708c65b09aea40989e9f
Reviewed-on: http://review.couchbase.org/34207
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>