Ready: Strong consistency update

- [x] Configs in `advanced.config`
- [x] `riak-admin` interface for ensemble
- [x] Implementation details

Heads up to @andrewjstone, @engelsanchez, and @jtuple when he's back.
Change to something like `Riak responds to the client after the first roundtrip which writes the data, but before the second roundtrip required to update the followers' Merkle trees, allowing the merkle tree update to happen asynchronously in the background;`