Refactor hot region related modules

[nolouch]

Motivation

We recently encountered a scheduling problem caused by the statistics module, see the issue for details: #4390

statistics is an important module for scheduling, it is basic and important. I see some problems:

• Difficult Maintain. For example, hot cache statics have many flags and states, it's hard to maintain.
• Lack Test. This module is best to prevent problems through a unit test. the unit test should be enhanced.
• More Robust. Statistics is also the sensor in the feedback system. we should make it more robust even the heartbeat frequency is changed.

[nolouch]

related task:

• Reduce the flag of HotPeerStat #4463 @rleungx
• statistic: refactor inherit in hot statistic #4510 @lhy1024
• statistics: add more test for hot statistics #4499 @lhy1024
• statistics: small refactor of hot statistics #4461 @rleungx
• scheduler: refactor hot region scheduler #4488 @rleungx
• metrics: refactor the hot region metrics #3726 @rleungx
• statistics: refacter store load in hot region scheduler #4149 @rleungx
• statistics: reorder the functions and clean up #4484 @rleungx

[lhy1024]

@rleungx Can we close it?

[rleungx]

Close it since it has finished.