Fixes RDF export of subobjects

Adds SemanticData from subobjects to the RDF export fixing https://bugzilla.wikimedia.org/show_bug.cgi?id=48708

Note: This is a partial integration of the patch https://gerrit.wikimedia.org/r/#/c/70047/4
Credits to Dbeeson



Code changes in short:
The Export code for RDF now understands that SemanticData has recursive information about subobjects and tries to iterate over them to add to the RDF export.
There are several ways of fetching `SubSemanticData` some with and others without the need to change interfaces. #366 chooses the least invasive method and extends `Sql3StubSemanticData` so that something like below will now yield `SubSemanticData` in one go without doing a custom `addSubSemanticData`. (Data are only loaded on request as lazy load).

```php
$semanticData = $this->getStore()->getSemanticData( $subject );

foreach ( $semanticData->getSubSemanticData() as $subSemanticData ) {
	// doing something with $subSemanticData
}
```

This should help to decrease the complexity of this code part.