cubicweb-dataio #3749320 Rdf store and external uris [validation pending]

It should be possible to avoid storing the external uris dictionnary (for memory reason).

prioritynormal
typebug
done in0.5.0
load0.200
load left0.000
closed by#40773c31df50 [dataimport] Do not store external uris dict, closes #3749320
patch[dataimport] Do not store external uris dict, closes #3749320 [applied]