br: initial scan is abnormal and data inconsistency when stop pitr task and start a same-name task in short time

[AkiraXie]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. 01.05 stop task
2. 01.05 start same-name task in short time ,with start-ts in 01.04

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

1. initial scan is normal
2. data consistency

3. What did you see instead (Required)

1. checkpoint lag increase a bit
   
2. initial scan thoughput increase a bit
   
3. data inconsistency

4. What is your TiDB version? (Required)

1. br --version
   Release Version: v6.6.0-alpha
   Git Commit Hash: 702a559
   Git Branch: heads/refs/tags/v6.6.0-alpha
   Go Version: go1.19.3
   UTC Build Time: 2022-12-31 11:41:31
   Race Enabled: false

[AkiraXie]

/component br
/severity critical
/found automation

[joccau]

/assign