log backup advancer stop the log backup task when it failed to get global checkpoint ts at the first tick

[Leavrth]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Another TiDB node becomes new log backup advancer owner.
2. the --start-ts of the log backup task is recorded as current global checkpoint ts at the first.
3. the advancer failed to collect the whole region level checkpoint ts, so it can't update the new global checkpoint ts.
4. it compares the current ts with its recorded global checkpoint ts(--start-ts), and finds that the gap is so large.
5. Therefore, the advancer stop the log backup task.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)