MB-9715: Add UPR Producer connection pausing

In the current code the upr thread continually calls the step
function for any producer connection. This change allows an UPR
Producer 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 Producer

Change-Id: I68f6cbd098ad3ed26ef2024ae2af7c032325da64
Reviewed-on: http://review.couchbase.org/34206
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>