v6.0.8

Use multi transactions to increase safety with delayed jobs (#318)

We now uses Redis transactions (with multi) when enqueuing a delayed job and deleting a delayed job queue.

We need to invoke the "watch" command by the scheduler to be notified of any changes to the delayed queue we are about to delete. Then we check that its length is 0. If it isn't we exit. If it is, we start preparing the new multi() command to delete the queue and its entry in delayed_queue_schedule. If a new job is inserted while we are doing this, the "watch" command will prevent the delete from succeeding.

Otherwise, even in the race condition when a job is being added to the same delayed queue that is being removed, the scheduler will remove the queue and entry in delayed_queue_schedule, and then the enqueue will add it back again.