Fix and cleanup validator status updates

[JustinDrake]

1. Replace the code under # Exit validator with a call to exit_validator.
2. Only remove a validator from the persistent committees and update validator_registry_delta_chain_tip if there's an actual exit event (i.e. EXITED_WITH_PENALTY or EXITED_WITHOUT_PENALTY).
3. Abstract away the grunt work around status changes (e.g. regarding validator_registry_delta_chain_tip, status, latest_status_change_slot) into an update_validator_status function, and replace the code under # Activate validator with that