Log Backup being paused due to a false positive in the automatic lag check.

[3pointer]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. start a log backup
2. wait for 48h(the default lag check threshold)
3. restart tidb advancer(or other scenarios that make advancer owner changed)

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

a normal log backup status.

3. What did you see instead (Required)

log backup paused by chance.

(The chance is the lastCheckpoint cannot be updated in time before checking. e.g. regions has a hole will make lastCheckpoint skip update this time and wait for next tick. )

4. What is your TiDB version? (Required)

v6.5.9/v7.1.5/v8.1.0/master