[dataimport] massive_store: do not drop constraints multiple times It is now done in __init__.

[dataimport] massive_store: do not drop constraints with `cascade` since it breaks things We want to be able to restore constraints at the end of the import. If they are silently dropped due to a CASCADE, we can't do that.

[dataimport] massive_store: drop all constraints in __init__ and restore all in finish This should avoid dependency issues when dropping specific constraints (e.g. when dropping a primary constraint referenced by a foreign key constraint). We only do this in master mode, slave mode stores are supposed to work with an initialized db already.