Continuously update the stats_meta of `mysql.tidb` in auto analyze loop

[jackysp]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. start a cluster
2. set tidb_slow_log_threshold to 0
3. check the query like update mysql.stats_meta set version = 437916222248714250, count = count + 0, modify_count = modify_count + 534 where table_id = 17

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

No this kind of query, because mysql.tidb hasn't been updated.

3. What did you see instead (Required)

stats_meta updated.

4. What is your TiDB version? (Required)

c21dc26