Sql3StubSemanticData to load SubSemanticData on request

- Add `__sob` as datatype to identify subobjects
- If `SubSemanticData` are requested by `Sql3StubSemanticData::getSubSemanticData()` then data are loaded on request only from the StorageEngine
- `hasSubSemanticData`, `findSubSemanticData`, and `getSubSemanticData` will initiate the lazy load
Given this is in a class named `StubSemanticData`, it's kinda evil to name a method `subSemanticData`, with different meaning. I initially misread the name of this method.