[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
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc