cubicweb-dataio #2730737 MassiveObjectStore: check and handle unicode strings in relate_by_iid [open]

relate_by_iid does not accept unicode-formatted from_eid and to_eid, whereas create_entity in session needs unicode-formatted arguments. This should be uniformized: perhaps, unicode should be required everywhere, in session as well as in all stores, in the whole API.

priorityimportant
typebug
done in<not specified>
load1.000
load left1.000
closed by<not specified>