cannot fast exit when resign leader

[nolouch]

Bug Report

What did you do?

When the store's number is 180. we resign pd leader may need one minute or more to finish elect the new leader:

details replay: https://gist.github.com/disksing/a1bf78007b674da7e15b52a9e25eb3a5

What did you expect to see?

a new leader elected in 10 secend.

What did you see instead?

1 minute

What version of PD are you using (pd-server -V)?

4.0.x, master