CBQE-0: using self.num_items instead of self._num_items

Change-Id: I43dc721e9553fd0baf9d84c794015d4999462531
Reviewed-on: http://review.couchbase.org/34996
Tested-by: Aruna Piravi <aruna@couchbase.com>
Reviewed-by: Aruna Piravi <aruna@couchbase.com>