Migrate ensure_computing transitions to new WorkerState event mechanism

[fjetter]

The task executing transitions should be migrated to the WorkerState event mechanism as outlined in #5736 (comment)

• The Worker.execute method is modified such that it no longer performs any transition but instead returns appropriate StateMachineEvents that trigger the necessary handlers. For instance
  • TaskFinished
  • Rescheduled
  • TaskErred
• The ensure_computing method is removed from the list of handle_comm every_cycle callbacks
• The logic of ensure_computing is moved to a private method (perspectively a method of the new class WorkerState)
• The new private method will not perform any transitions but rather return a list of recommendations
• This private method is then called as part of the transition system, e.g.
  • unpause handler
  • transition_waiting_ready
  • transition_executing_*
• The implicit goal is to remove all invocations of self.loop.add_callback(self.execute, ...) and replace this with the new callback method

Blocked by

• StateMachine event dispatch mechanism #5894

Related work

• Encapsulate spill buffer and memory_monitor #5891
• Migrate ensure_communicating transitions to new WorkerState event mechanism #5896

[crusaderky]

Closed by

• Migrate ensure_computing transitions to new WorkerState event mechanism - part 1 #6003
• Migrate ensure_computing transitions to new WorkerState event mechanism - part 2 #6062
• Log state machine events #6092
• Cancel asyncio tasks on worker close #6098