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

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>
(Imported from commit c9664b4b17660c03ed96072a9f5392dbb0800f2c)