Implement log compaction

[andreev-io]

Section 7 of the Raft paper describes a much-needed optimization: log compaction. With it, the cluster will be able to snapshot its state from time to time so that replicas in the future don't have to replicate the entire transition log and so that storage space is preserved.

Implementing this in Little Raft according to the paper is a good way to make Little Raft a production-grade library. This is a good feasible challenge for people that want to build important functionality of a real distributed system following a clear specification (the Raft paper section 7, in this case).

[andreev-io]

I'm working on this.