Consider priority to fix the Hot Region

## Feature Request

### Describe your feature request related problem
In some situations, the hot region operator may fail, such as add a learner then timeout. Hot Region will not schedule the region again because the region is unhealthy with an unexpected peer number. but the region may always not fix in a long time, which lead the hot issues.

### Describe the feature you'd like
- Consider priority to fix the Hot Region
- Figure out some reason about the long unhealthy status.

### Describe alternatives you've considered

The hot region schedule needs to consider the unhealthy region.

### Teachability, Documentation, Adoption, Migration Strategy
- GO