potential memory leak when modify password

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

I create 2M users, and then call "alter user 'xxx' identified by 'yyy'" for every user.

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

No memory leak.

3. What did you see instead (Required)

The memory usage is abnormal

I add a internal sessions metrics and it shows that the internal session leaks:

Profile also shows the ExecuteInternal takes a lot inuse memory:

4. What is your TiDB version? (Required)

master ... this should affect a lot versions.

[lance6716]

/assign @lance6716