MB-9661: API to count changes between two sequence numbers

Change-Id: Icf62266bf29b5981d1e9dd6d33aa784a5a90d04f
Reviewed-on: http://review.couchbase.org/32412
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>