[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html