Add nru parameter to the upr mutation api

Change-Id: Ib343d1ff5b15f67aebd1c144b2890e64af10de66
Reviewed-on: http://review.couchbase.org/31795
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>