Enhancement

For now, we didn't consider region status when dispatch MPPTask(tidb or tiflash_compute node). It may lead to MPPTask send to node whose region snapshot has not been synced from tikv, which make query to wait for a long time(aka wait index timeout, which is 10s).

So we need to consider region's pending status.

CN didn't consider region status at all:

TiDB only consider down peer, didn't consider pending peer: