Handle the timeout operator with pending learner with a high priority

## Feature Request

### Describe your feature request related problem

We have a timeout for operators that mark OpRegion. The default value is 10 minutes. https://github.com/tikv/pd/blob/6dc21259f151f1447bdddff5d9951e8f9a2d551d/server/schedule/operator/operator.go#L38

If the learner is added, but cannot catch up with the log and be promoted to the follower for 10 minutes. It's usually something wrong somewhere.

### Describe the feature you'd like

We can add this region into suspect regions to handle it with high priority.

https://github.com/tikv/pd/blob/c3a15b31b4276e7325587dc784fb1197a98e244d/server/cluster/cluster.go#L106

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy