Tundra: add S3 DataStore, skeleton disk & Carmine DataStores
~Tundra: completely refactor to use standard mq worker

Pros:
* Significantly simpler.
* More robust (uses trusted, tested mq for heavy lifting).
* Inherits mq features like backoff.
* Allows better datastore work distribution (freezing isn't done in batches).
* Easier datastore protocol extension (works on single keys rather than maps).

Cons:
* None really.
~Tundra: refactor Faraday impl for new API
README: Clean up Tundra docs, mention new S3 impl, alpha->beta