Tundra: completely refactor to use standard mq worker

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

Cons:
* Old batching method -may- be preferable against certain stores.
Tundra: refactor Faraday DataStore for new API
Tundra: add S3 DataStore, skeleton disk & Carmine DataStores
README: Clean up Tundra docs, mention new S3 impl, alpha->beta
Tundra: new unit tests
v2.4.0-beta2