Add a stat to allow upr takeover estimates

Change-Id: I3de6c22c1b399daa69f22c0db626580a21f08810
Reviewed-on: http://review.couchbase.org/33166
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>