[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.

cubicweb-web: embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js