TTL should force to kill the scan SQL when TTL job cancels

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. create a TTL table and insert a big number of rows but none of these rows is expired.
2. wait TTL job start
3. cancel the TTL job by set @global.tidb_ttl_scan_worker_count = 0

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

The scan SQL should be killed immediately.

3. What did you see instead (Required)

The TTL will wait for the SQL done even if the TTL job cancelled.

4. What is your TiDB version? (Required)