Implement snapshot creation policy

[jopemachine]

The current implementation of creating a snapshot is only executed after handle_committed_normal_entry.

I think there could be better ways.

For example, we can implement the following policies:

Create a snapshot when the size of the FSM exceeds a specific threshold.
Generate a snapshot when the number of log entries surpasses a certain count.

Note: snapshot contains several metadata configuring the cluster, so wrong implementation of this would break the consistency of the raft loop.