Make StateMachine snapshotting methods optional by moving them to separate trait with default implementation

[andreev-io]

Snapshotting in Raft is optional. However, Little Raft enforces that all users of the StateMachine trait implement get_snapshot, create_snapshot, and set_snapshot; the implementation can be a no-op if the user doesn't want to use snapshotting, but the code doesn't make that clear.

We should figure out how to move snapshot-related methods into a separate trait that has a default no-op implementation that Little Raft users can use if they choose to avoid shapshotting.