A consensus library that supports creating multiple consensus groups
(ensembles). Each ensemble is a separate Multi-Paxos instance with its own
leader, set of members, and state.
Each ensemble also supports an extended API that provides consistent key/value
operations. Conceptually, this is identical to treating each key as a separate
Paxos entity. However, this isn't accomplished by having each key maintain its
own Paxos group. Instead, an ensemble emulates per-key consensus through a
combination of per-key and per-ensemble state.
