Sql3StubSemanticData to load SubSemanticData on request

- Add `__sob` as datatype to identify subobjects
- If `SubSemanticData` are requested `Sql3StubSemanticData::getSubSemanticData()`, data are loaded on request only from the StorageEngine
- `hasSubSemanticData`, `findSubSemanticData`, and `getSubSemanticData` will initiate the lazy load