We track last-seen dates for task queues, but not for workers -- just a first-claim and an expiration.

That leads to confusion such as https://bugzilla.mozilla.org/show_bug.cgi?id=1690950 where there's no useful data to indicate that some of those workers are no longer doing anything.

Let's add a last_date_active field to workers as well, and update that on claimTask, claimWork and reclaimTask.

This should have the same kind of client-side rate-limiting that the task_queues.last_date_active does, but with a shorter timeframe (perhaps 30 minutes?), so that any last_date_active less than 30 minutes ago should be considered "current".

Displaying this in the UI is a separate project.