cubicweb-dataio #2550573 Massive store - indexes and cnxs [validation pending]

The massive store drop and create index before inserting the meta data. It may be interesting to make this optional, in order to do this only once when using multiple stores.

Additionaly, the cnxs should be properly managed, in order to avoid an empty cnxs set after multiple commits.

done in0.1.0
load left0.000
closed by#4b2736f5ee10 [dataimport] Drop and create constraints once for all (closes #2550573), #dfc49c4e0a66 [dataimport] Use self.commit() and no more self.session.commit() (closes #2550573). This (seems) to remove the empty cnxset problems.