Carbonado Compare and Swap Hash

Provide `&cash=` parameter with Carbonado bao hash to avoid overwriting old versions of the stash. This can help prove as a prototype for this behavior for carbonadod, also.

This will likely require changes to the client also, since if it's using an old hash for that file, it will need to be refreshed. It will also need a BTreeMap to maintain the latest list of known hashes.

I'm still on the fence on this approach, since I'm not sure it solves every concurrent overwrite issue. Ideally we would use something like automerge.