Move more sql construction responsibilities into the DV handlers

No and yes. We need a field for that but it should be called `value_identity` according to https://www.mediawiki.org/wiki/Extension:Wikibase_QueryEngine/SQL_schema_proposal, maybe `identity` or `hash`. And it should always contain the `DataValue->getHash()`.

The data value handlers should not reimplement an other getHash method. This was what confused me in the first place.