Implement shared cluster state with consensus for changes

[psFried]

This is a big issue, but will unblock a lot of others once it's working. We need some way to have shared mutable state in the cluster. Use cases for this are:

• max number of events - This has to be consistent for all the nodes in the cluster
• Automatically determining/assigning actor_ids to server nodes
• Adding the concept of users and authentication/authorization - We have to be able to define a user and grant them roles. We don't want to have to create users on each individual server
• This could also be used for more far-off ideas like consumer groups

One idea for how to implement this is to just have a /system namespace that can be used by the cluster itself to produce events into. We could then implement raft by requiring acknowledgement of the events by a majority of nodes.