Improve the process of making decision by utilizing statistics

## Development Task
Consider that we are going to rely on PD to make a decision about if we need to scale out/in. It's better to utilize the PD's advantage to help make a decision. For example, PD has a global view. It knows the topology of the system and has the statistics of TiKV.

As for topology, we can utilize it to decide where TiKV/TiDB should be scaled out. And for statistics, we can use it to solve the hotspot problems.