can't kill query for CHECK_CONSTRAINTS

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

mysql> kill 1152615042;
Query OK, 0 rows affected (0.20 sec)

mysql> show processlist;
+------------+------+-------------------+--------------------+---------+------+------------+----------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+------------+------+-------------------+--------------------+---------+------+------------+----------------------------------------+
| 1152615042 | root | 10.138.0.78:1042 | information_schema | Query | 289 | autocommit | select count(*) from CHECK_CONSTRAINTS |
| 1152615016 | root | 10.138.0.78:16990 | NULL | Query | 0 | autocommit | show processlist |
+------------+------+-------------------+--------------------+---------+------+------------+----------------------------------------+
2 rows in set (0.20 sec)

### 2. What did you expect to see? (Required)
The query is killed.

### 3. What did you see instead (Required)
The query is still running.
### 4. What is your TiDB version? (Required)
master