[dataimport] _create_copyfrom_buffer: fix datetime converter + add time converter
Include second and microsecond in the output instead of dropping them on the
floor. The time zone is not supported for now, though.
Related to #3845572
[dataimport] _create_copyfrom_buffer: raise ValueError if conversion cannot be performed
If the conversion fails, there is no reason to continue execution. One should
notify the error.
Continuing after a misconverted data could corrupt the database.
Related to #3845572