_leader_lease should also be considered when handling vote

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.

Start by locating the vote-handling path and the code that manages _follower_lease and _leader_lease. Read how step_down invalidates _leader_lease before a vote, then trace the idle-group and returning-follower cases described here. Done means vote handling accounts for the valid leader lease without introducing leadership-change latency jitter.

Written by the indexing model from the issue text.