cubicweb #2450680 storage may not properly delete value is the entity were edited in the same transaction [validation pending]

we should explicitly detect deletion instead of trying to guess this is a deletion from the lack of edition specific attribute

prioritynormal
typebug
done in3.14.9
load left0.000
closed by#b922dd08eb79 [storage] relies on event=='deleted' to detect "delete" event (closes #2450680)