Cache item maybe missed in hot peer cache

[lhy1024]

If the region is balanced from a store to another store or the leader is transferred, the region and store info will be expired in hotPeerCache.

We statistic the flow by comparing the current time with the previous time, which will cause the existence of zero in the previous time after balance or transfer. In order to avoid sudden fluctuations, we chose to use the region info of other stores as the last time.

But tmpItem will be affected by the expired store. if there are store1, store2, and store3, and move peer to store4, the new replica cannot get cached from all unexpired stores.