[crrf]inject io hang to the pd leader, the upstream checkpoint continued to move, but the downstream checkpoint stopped for about 18 minutes

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

1. enable crrf
2. inject pd lead io hang block_io_delay(volumeMountPath:/var/lib/pd,containerName:pd,readLatency:10m,writeLatency:10m)
3. check upstream and downstream checkpoint

### 2. What did you expect to see? (Required)
downstream and upstream checkpoint can continued to move

### 3. What did you see instead (Required)
upstream checkpoint keep moving
but downstream checkpoint stops for 18 minutes

### 4. What is your TiDB version? (Required)

crrf test version