Deadlock: tasks stolen to old WorkerState instance of a reconnected worker #6356

Copied from #6263 (comment)

1. Stealing decides to move a task to worker X.
   1. It queues a steal-request to worker Y (where the task is currently queued), asking it to cancel the task.
   2. Stores a reference to the victim and thief WorkerStates (not addresses) in WorkStealing.in_flight
2. Worker X gets removed by the scheduler.
3. Its WorkerState instance—the one currently referenced WorkStealing.in_flight—is removed from Scheduler.workers.
4. Worker X heartbeats to the scheduler, reconnecting (bug described above).
5. A new WorkerState instance for it is added to Scheduler.workers, at the same address. The scheduler thinks nothing is processing on it.
6. Worker Y finally replies, "hey yeah, it's all cool if you steal that task".
7. move_task_confirm handles this, and pops info about the stealing operation from WorkStealing.in_flight.
8. This info contains a reference to the thief WorkerState object. This is the old WorkerState instance, which is no longer in Scheduler.workers.
9. The thief's address is in scheduler.workers, even though the theif object isn't.
10. The task gets assigned to a worker that, to the scheduler, no longer exists.
11. When worker X actually shuts itself down, Scheduler.remove_worker goes to reschedule any tasks it's processing. But it's looking at the new WorkerState instance, and the task was assigned to the old one, so the task is never rescheduled.