cubicweb #1806937 range facet optimisation [resolved]

we could retrieve only min/max values, this is the only information currently used by associated widget (slider)

prioritynormal
typeenhancement
done in3.13.1
load0.300
load left0.000
closed by#58f11f7dad68 [range facet] closes #1806937: simply retrieve min/max values when computing vocabulary/possible_values, this is enough