[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.
from cubicweb.schemas.base import in_group, CWSourceSchemaConfig
# copy __permissions__ to avoid modifying a shared dictionary
in_group.__permissions__ = in_group.__permissions__.copy()
in_group.__permissions__['read'] = ('managers',)
cw_for_source = CWSourceSchemaConfig.get_relation('cw_for_source')
cw_for_source.__permissions__ = {'read': ('managers', 'users'),
'add': ('managers',),
'delete': ('managers',)}