[dataimport] Add a ``flush`` method for all stores. For ``ObjectStore``, ``RQLObjectStore`` and ``NoHookRQLObjectStore`` the method tries to call the ``commit`` method. For ``SQLGenObjectStore``, the ``flush`` method is basically unchanged with respect to previous changesets.

#!/usr/bin/env python
from cubicweb.cwctl import run
import sys
run(sys.argv[1:])