flush statement should trigger implicit commit

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

For example:

begin;
insert into t values (1);
flush privileges;
rollback;

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

select * from t should see 1 is inserted.

3. What did you see instead (Required)

1 is not inserted

4. What is your TiDB version? (Required)

master ffa4ba9