ttl log "fail to lock scan task" with error "didn't find table with id"

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. create a new TTL table
2. wait TTL job started and grep "fail to lock scan task" in logs

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

Normally, we should not have some warn logs.

3. What did you see instead (Required)

Some warns logs found:

[task_manager.go:331] ["fail to lock scan task"] [ttl-worker=job-manager] [ttl-worker=task-manager] [jobID=5fb0a12319c640908224910c43836a06] [scanID=0] [tableID=284] [error="didn't find table with id: 284"]

4. What is your TiDB version? (Required)