Nimisha/split converge api

I think the manifestation would be that we would have multiple copies of the same child.  And then the UI would render one or the other.  If it's not deterministic in what it renders, we would have flaky behavior.  (Or, I may be completely off about the UI.)

Regardless, it looks like the underlying infrastructure (modulestore or xBlocks) doesn't enforce single-copies of a child.