region cache contain the down peers

[lzmhhh123]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

use the follower read or tiflash read then make one peer down manaully.

2. What did you expect to see? (Required)

tidb don't access the down peer.

3. What did you see instead (Required)

there are lots of error logs in tidb-server, because it accesses the down peer.

4. Affected version (Required)

v4.0.0-RC.2

5. Root Cause Analysis

PD should tell tidb the down peers and tidb should filter it.