MB-7486: Couchstore stats should be resetable

Change-Id: I46e4fc3b1003caf3602e542467c3e7590388ebb2
Reviewed-on: http://review.couchbase.org/23694
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>