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.
I'd change this paragraph to: `Strongly consistent operations on a given key may also fail if a majority of the peers in the ensemble group for that key are not available due to slowness, crashes or network partitions. To decrease the probability of data unavailability, it serves to increase the number of nodes such that a smaller percent  of them are likely to fail at any given time. Furthermore, this spreads out the key space across nodes such that the possibility that any given node affects a large number of keys is reduced. For these reasons, among others we recommend using strong consistency in larger clusters. More on that in the section on [[fault
+tolerance|Managing Strong Consistency#Fault-Tolerance]]`