This issue is a tracker for the worker state machine refactor, as described in the wider epic #5736.

In scope for this issue is to yank out all state machine logic from Worker and create two new classes, WorkerState and BaseWorker.

Writing additional unit tests, beyond tweaking the already existing ones, is out of scope.

Contributing PRs:

• Deprecate WorkerState accessors #6579
  Blocked by:
  • Refactor wait_for_state() #6581
  • Yank state machine out of Worker class #6566
    Blocked by:
    • Migrate ensure_communicating transitions to new WorkerState event mechanism #5896
    • Refactor all event handlers #6410
    • Encapsulate Worker.batched_stream.send() #6475
      Blocked by:
      • @fail_hard can kill the whole test suite; hide errors #6474
    • Don't invoke log_event from state machine #6512
    • Make worker state machine methods private #6564