Add necessary methods to support finding nodes/versions resulting from a split

Can be used in a transactional system to update the absent set after a write-my-reads insert that results in a split.

I can squash the commits to make the git log nicer?
Nits, but what the hell:
The identifier style here would be
* newnodes -> new_nodes
* newnodes_vector_type -> new_nodes_type

Then maybe given the comment above, we should use other names? like