Release event hooks when process is terminated

[chrisjsewell]

The three referencing lambda functions I identified in aiidateam/aiida-core#4603 (comment) are from plumpy.Process._setup_event_hooks (

plumpy/plumpy/processes.py

Lines 318 to 330 in 848028b

	def _setup_event_hooks(self) -> None:
	"""Set the event hooks to process, when it is created or loaded(recreated)."""
	self.add_state_event_callback(
	state_machine.StateEventHook.ENTERING_STATE,
	lambda _s, _h, state: self.on_entering(cast(process_states.State, state))
	)
	self.add_state_event_callback(
	state_machine.StateEventHook.ENTERED_STATE,
	lambda _s, _h, from_state: self.on_entered(cast(Optional[process_states.State], from_state))
	)
	self.add_state_event_callback(
	state_machine.StateEventHook.EXITING_STATE, lambda _s, _h, _state: self.on_exiting()
	)

).
So I imagine these event hooks need to be removed, once the process enters a terminal state.

Originally posted by @chrisjsewell in aiidateam/aiida-core#4603 (comment)

[chrisjsewell]

Also noted by @muhrin

Yes, good point! All those callbacks should be cleared on termination. I'm also wondering if there are any situations where a process has registered itself with the communicator for callbacks but doesn't un-register....