_leader_lease should also be considered when handling vote

[BusyJay]

Currently only _follower_lease is considered, and leader will never update _follower_lease, so _follower_lease will always expire on leader side. If a follower starts election in an idle group, leader will vote for follower even the leader lease is still valid. Leader will call step_down to invalidate _leader_lease before vote, so there seems to be no correctness problem. But if a follower is isolated for a while and then return, it's likely to cause leadership change and lead to latency jitter on the client side.