Remove validators from persistent committees after exit

[JustinDrake]

At the moment exit_validator removes a validator from the persistent committees when updating the status ACTIVE_PENDING_EXIT. We should wait for an exit event (EXITED_WITHOUT_PENALTY or EXITED_WITH_PENALTY) to remove a validator from the persistent committees.

[djrtwo]

pulled this list from #253

• Replace the code under # Exit validator with a call to exit_validator
• Only remove validator from 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).
• Abstract away all 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.

[djrtwo]

ah, closing in favor of #274