Sql3StubSemanticData to load SubSemanticData on request

- Add '__sob' as datatype to identify subobjects
- Add `Sql3StubSemanticData::getSubSemanticData`
- Add `Sql3StubSemanticData::hasSubSemanticData`
Merge pull request #366 from SemanticMediaWiki/store-sobj

Sql3StubSemanticData to load SubSemanticData on request