Need update TiFlash replica available value when some of regions is down or pending.

## Enhancement
- If some of the regions in TiFlash sync very slow.
- Then in TiFlash, The current peer may report itself as a `down-peer` or a `pending-peer`
- On the TiDB side, it still will see the TiFlash replica available as `true`. So the query may still send to the TiFlash.
- But in this time, lots of regions have not be synced, and The query in TiFlash will got `region timeout error`