Aquire and release a reference for all upr connections

If we want to be able to notify upr connections when they are
paused then we need to reserve the connection in memcached.

Change-Id: I4ae6097cf211b7c55f3fa8aef92eaa1f9ba8ab14
Reviewed-on: http://review.couchbase.org/34110
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>