ctdb-locking: Schedule the next possible lock based on per-db limit

ctdb-locking: Schedule the next possible lock based on per-db limit

This prevents searching through active lock requests for every pending
lock request to check if the pending lock request can be scheduled or not.
The locks are scheduled in strict first-in-first-out order.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Volker Lendecke <vl@samba.org>