ebs br: restore could hang if some tikv nodes are killed or restarted

[BornChanger]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Kill some TiKV node during ebs br restore phase

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

EBS BR restore continue and succeed

3. What did you see instead (Required)

EBS BR restore hangs

4. What is your TiDB version? (Required)

TiDB 6.5 and above

[BornChanger]

/assign @YuJuncen