After a snapshot is successfully created (per #83), remove committed log entries leading up to that state. Snapshot procedure described in Section 7 of the extended Raft paper: https://raft.github.io/raft.pdf

This can be done safely, independent of leadership because it is not possible for prior entries to be invalidated after being committed.

Dependent on #111 to ensure that servers do not lose data on restart after a snapshot is taken