leader can still accept read requests when transferring leader

[BusyJay]

Because new leader has to disrupt the old leader before it can receive votes from other candidates, so old leader will always has the latest data before it votes for the new leader and in lease. So it's safe for old leader to accept read requests. Though it should not accept write requests to avoid breaking transfer leader.

The current implementation will call _leader_lease.on_leader_stop(), which will expires lease and prevent accepting any read requests.