offline_stats can not be updated after merge offline peer

[lhy1024]

Bug Report

What did you do?

offline some store

What did you expect to see?

the num of offline peer is zero after offline store is finished

What did you see instead?

there are still some offline peer after offline store is finished

use pdctl to query these regions and the result is null

And we also found their last operation is merge region

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

5.0.0

[lhy1024]

I think it may be better to update offline stats when updating region cache because of merging region