Table creation becomes very slow when there are many foreign key tables

[GMHDBJD]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create fk tables concurrently

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

table created fastly

3. What did you see instead (Required)

Table creation becomes very slow when there are many foreign key tables

4. What is your TiDB version? (Required)

master

[wjhuang2016]

It's expected for now, so I changed it to enhancement.