locking: Do not create multiple lock processes for the same key

locking: Do not create multiple lock processes for the same key

If there are multiple lock helper processes waiting for the same record, then
it will cause a thundering herd when that record has been unlocked.  So avoid
scheduling lock contexts for the same record.  This will also mean that
multiple requests will get queued up behind the same lock context and can be
processed quickly once the lock has been obtained.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
(This used to be ctdb commit ebecc3a18f1cb397a78b56eaf8f752dd5495bcc9)