fix: 'types.OkResult' is not a valid value type for 'int'

[fanyang01]

This PR resolves the following error:

mysql> UPDATE tb1 SET value=3.3 WHERE id=1;
ERROR 1105 (HY000): error: 'types.OkResult' is not a valid value type for 'int'

The root cause is that we return types.OkResultSchema for DMLs, which breaks go-mysql-server's assumptions. I have modified our fork of go-mysql-server to be more robust.

[NoyException]

LGTM, great work!