refresh-who-has can break the worker state machine #6528

• Blocks Release 2022.6.0 community#252

This is a regression introduced in #6342 + #6348.

1. Two tasks, x and y, are only available on a busy worker.
   The worker sends request-refresh-who-has to the scheduler.
2. The scheduler responds with refresh-who-has, stating that x has become missing, while y has gained an additional replica.
3. The handler for RefreshWhoHasEvent empties x.who_has and recommends a transition to missing.
4. Before the recommendation can be implemented, the same event handler invokes _ensure_communicating to allow y to transition to flight. This in turn pops x from data_needed - but x has an empty who_has, which trips an AssertionError in _ensure_communicating.
5. This in turn trips @fail_hard, which violently shuts down the whole worker.