cubicweb #5040344 [dataimport] define and/or update the store API [validation pending]

It looks like we currently have the following common denominator:

.create_entity(etype, **args) -> entity
.relate(eidfrom, rtype, eito) -> None
.flush() -> None

We propose (as discussed in the last copil):

.prepare_entity(etype, **args) -> eid
.prepare_relation(eidfrom, rtype, eidto) -> None
.flush() -> None

done in3.21.0
load left0.000
closed by<not specified>
patch[dataimport] provide a new clean API for dataimport [rejected][dataimport] Move stores to new API. [applied][dataimport] Move stores to new API. [folded]