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.
The corner cases aren't really rare. I'd change the last clause to `which can happen any time `target_n_val` is greater than the number of physical nodes or the `nval` configured for a bucket is greater than `target_n_val`.