[dataimport] Move stores to new API. Here is the final store API: * ``prepare_insert_entity(<entity type>, **kwargs) -> eid``: given an entity type, attributes and inlined relations, return the eid of the entity to be inserted, *with no guarantee that anything has been inserted in database*, * ``prepare_update_entity(<entity type>, eid, **kwargs) -> None``: given an entity type and eid, promise for update given attributes and inlined relations *with no guarantee that anything has been inserted in database*, * ``prepare_insert_relation(eid_from, rtype, eid_to) -> None``: indicate that a relation ``rtype`` should be added between entities with eids ``eid_from`` and ``eid_to``. Similarly to ``prepare_insert_entity()``, *there is no guarantee that the relation will be inserted in database*, * ``flush() -> None``: flush any temporary data to database. May be called several times during an import, * ``finish() -> None``: additional stuff to do after import is terminated. **Warning:** ``prepare_update_entity()`` still needs to be implemented for NoHookRQLObjectStore. Related to #5040344

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