Reduce the number of hot region scheduler configurations

## Feature Request

### Describe your feature request related problem

### Describe the feature you'd like

There are lots of configurations we never use. Or we are even not very clear about how will them affect the system. These kinds of configurations can be changed to a constant, e.g., `max-peer-number`, `max-zombie-rounds`.

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy