cubicweb-dataio #3047120 make drop index optionals in MassiveObjectStore [validation pending]

The store systematically drops index, inserts data and rebuilds index.

The decision of dropping index should be left to the client code since it might not be the best choice. It might even not have permissions to do so.

done in0.2.0
load left0.000
closed by#721b76c02a64 make drop index optional in store (closes #3047120)
patchmake drop index optional in store (closes #3047120) [applied]