snap_br: possible inconsistency not reported while TiKV restarts.

[YuJuncen]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Start EBS snapshot backup.
2. During EBS snapshot backup, restart a TiKV node.

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

The backup should fail due to possible consistency break of suspending lightning.

3. What did you see instead (Required)

The backup successes.

4. What is your TiDB version? (Required)

Master.