when shrinking the TTL workers, some expired rows will left undeleted

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. create TTL table with many rows.
2. wait TTL job start
3. set @@tidb_ttl_scan_worker_count to a smaller value
4. wait TTL job done
5. check if all expired rows deleted

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

All expired rows should be deleted

3. What did you see instead (Required)

some expired rows are not deleted because when shrinking TTL worker, a task will be marked as finished.

4. What is your TiDB version? (Required)