Create a Storage trait

[andreev-io]

What happens when a new replica is added to the cluster? In the current design, other replicas won't learn about it unless they are restarted with the new peer_ids. But upon restart, the replicas and their state machines will lose their state unless the user builds some persistence themselves.

To make this simpler for the user, we should add functionality for the replica to preserve some permanent state. This should exposed to the Little Raft user via a trait that they can implement as they wish. The Raft paper has a lot of good info on what state needs to be preserved and how, so a good first step would be implementing that, and then adding functionality to snapshot the state of the state machine.