hot region: dynamic update the hot region threshold

## Enhancement Task
Currently, pd identifies the region as hot only when the flow of this region is bigger than the fixed threshold. But sometimes, there are many small flows in many regions; the store flow is big, but every region flow is small.

related code:
https://github.com/tikv/pd/blob/8bc2311c49dc2ccb2d30478e165804c315d05e57/pkg/statistics/utils/constant.go#L38-L46