unexpected continuous cross AZ traffic after tikv shutdown

[zyguan]

Let the cluster has 3 AZs and each AZ has 2 tikvs, then shutdown 1 tikv in AZ1. Since there is still one alive tikv in AZ1, it will hold all regions finally, thus replica reads with closest-replica from AZ1 can be performed on that tikv. However, the tidb from AZ1 still continuously access replicas from other AZs.

[zyguan]

fixed by #1122