Add support for updating cluster membership

The main necessary changes to make this happen:

Introduce a new kind of payload to the Raft log entry, holding configs
Teach the cluster module to deal with the possibility of cluster members changing
Add a new rpc to the Raft service to support changing cluster configurations
Teaching Raft cluster members to support the case of not being part of the cluster
Implement the notion of "joint consensus" and the two-phase config shift as described in the Raft paper

dinowernli / concord