Support not splitting regions on placement rules

## Feature Request

### Describe your feature request related problem

If a cluster is creating a lot of tables with tiflash, it will also split a lot of regions. Too many regions can lead performance regression and forbid the aggressive optimizations like separating DB for regions.

### Describe the feature you'd like

Don't split regions on placement rules, instead, scheduling all regions that is overlapped by the placement rules. If there are conflict, just reject setting the rules.

### Describe alternatives you've considered

@disksing suggests let TiDB produce a large enough range for all future tables. But TiDB can't forecast the table creation unfortunately. Besides, if users create a lot of tables, and apply rules one by one occasionally, it can still result in many regions.

### Teachability, Documentation, Adoption, Migration Strategy

The change is backward compatible except when there are conflict rules already, which should rarely happen.