Require the FFI to explicitly track revisions

[alarso16]

Rather unintuitively, in concurrent contexts, there is no way to guarantee that revision won't be removed from the revision manager during use, since Rust cannot accurately count the number of references in use. This is currently handled relatively gracefully by returning an error upon the next use, but it would be ideal if the FFI supported holding a revision in use, even if it could be evicted due to age.

[demosdemon]

@alarso16, am I doing what you asked here in #1158?

[demosdemon]

Ah, after rethinking, the answer is sort of but no. But, I have set up the framework for doing so. Instead of returning a pointer to a proposal, we can return a pointer to a nodestore.