TTL task will not be taken by other tidb after a failover

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. create TTL table

> set @@global.tidb_ttl_running_tasks=1;
> create table ttl(t timestamp default current_timestamp) TTL=`t`+interval 1 HOUR;

2. Run select * from mysql.tidb_ttl_task to check whether the task is scheduled and in running state, kill the current tidb.

3. Restart tidb to wait TTL job finished

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

The task should be taken by the restarted tidb and continue to run.

3. What did you see instead (Required)

The task will never recover and the TTL job will not finish forever

4. What is your TiDB version? (Required)

master