Subsubsite bugfix

had an issue with subsites within subsites, where the registry proxy failed with a maximum recursion error, because it always returned the same LineageRegistry instance.

this pull request fixes this bug.

+ fixed one wrong import of FieldRef, where the interface was imported instead of the implementation class.

@jensens please have a quick review... some more comments in the code
@jensens please don't merge yet. i run into some troubles.