Sql3StubSemanticData to load SubSemanticData on request

- Add `__sob` as datatype to identify subobjects
- Only if `SubSemanticData` are requested by `Sql3StubSemanticData::getSubSemanticData` data are loaded from the StorageEngine
- `hasSubSemanticData` and `getSubSemanticData` are adapted to care for the lazy load