Migrate queue's queue to a more visible form

[djmitche]

The queue uses tc-lib-azqueue for its core "queueing" operations. This supports both queues for tasks for workers, and delay queues for things like deadlines and task claims.

Because it mixes so many things in there, this queue is not easily introspected to find out, say, what tasks are pending for a particular task queue.

Let's find a way to do task queueing that's linked to the tasks table in a way that allows querying (#2939) the tasks in the queue. The pendingTasks API method should be upgraded in the process to take account of cancelled tasks (https://bugzilla.mozilla.org/show_bug.cgi?id=1434851).

Requires #2932.

[djmitche]

Note that postgres supports NOTIFY/LISTEN, allowing us to avoid polling (or for safety's sake, polling very infrequently) the DB when nothing is changing.

[lotas]

current thought progress documented in this gist https://gist.github.com/lotas/57779afe067e4f8eb586f37fe25016c9

[lotas]

Execution of this transition:

• Part 1: prepare azure_queue_messages table Prepare azure_queue_messages for upcoming migration. Part 1/3 #6550
• Part 2: migrate data from azure_queue_messages into separate tables, keep things running Queue internals refactoring Part 2/3 #6554
• Part 3: add "listen" functionality to avoid polling, drop azure_queue_messages

after that #2939 will add new functionality to introspect queues and change task priorities

[lotas]

I've extracted that last piece into as separate ticket and will close this one #6950