Get num backfill items from the kvstore

This should be much more accurate and I also found some
underflow issues with the old implementation.

Change-Id: I3f90c04dac0fbe103d4016e666a03edb31061eff
Reviewed-on: http://review.couchbase.org/30763
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>