Queue - When UserJob.queue_id works down to zero tasks, update status and fire hook

Queue - When UserJob.queue_id works down to zero tasks, update status and fire hook

Suppose you setup a queue with several tasks and then run them all. What happens
to the queue's status?

Before
------

The status always remains `active`.

After
-----

Depends on the use-case:

* If you have an open-ended queue providing an on-going service (no `UserJob`),
  then the status remains `active`.
* If you have a fixed-purpose queue attached to a `UserJob`, then the status
  changes from `active` to `completed`, and it fires an event:
    ```
    function hook_civicrm_queueStatus(CRM_Queue_Queue $queue, string $status)
    ```

Technical Details
-----------------

* There are two main ways that items get removed from a queue (`Queue.runNext`
  API and `civicrm/queue/ajax/runNext`).  Both of these fire an internal event
  (`civi.queue.check`) to consult the status.