Now `Datetime` support `Datime(6)` to indicate the time scale. For different database , the max scale is not different.
`Datetime` means `Datetime(0)`.
Reviewed-on: https://gitea.com/xorm/xorm/pulls/2021
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-committed-by: Lunny Xiao <xiaolunwen@gmail.com>