Rollback config in store when kv.persist failed

[bradyjoestar]

I am reading the following code
https://github.com/pingcap/pd/blob/f72cc23e5765b32c0d503e4fecd4b37fea44c98f/server/server.go#L531-L542
If s.scheduleOpt.persist(s.kv) returns err, it will not rollback to the old config in scheduleOpt

Maybe we'd better add the rollback code? just like this:

old := s.scheduleOpt.load() 
s.scheduleOpt.store(&cfg) 
if err := s.scheduleOpt.persist(s.kv); err != nil { 
        s.scheduleOpt.store(old) 
 	return err 
 } 

[nolouch]

@bradyjoestar good catch!

[bradyjoestar]

@nolouch I will create a new pr for these similar case. 😬