story 71751536. rdf:type triples are duplicated in output

Remove duplicates from RDFStream.

https://www.pivotaltracker.com/s/projects/684825/stories/71751536
1) I'm actually fine with duplicate triples. I understand @escowles's point, but it just doesn't bother me that much. RDF is a machine format.

2) If we can understand how to avoid the duplicates at some closer-to-the-JCR layer, that is without question the right thing to do. It shouldn't be that hard. It currently seems to be a matter of maintaining some in-flight state in the subclasses of NodeRdfContext.

3) NodeRdfContext is a piece of junk. In fact, the whole triple generation subsystem should be reworked to bring far more of the action out into the type system. And yes, I wrote a huge amount of that and that's one reason I'm so sure about it.