akka-raft now supports log compaction

Compaction is implemented via snapshotting, since https://github.com/ktoso/akka-raft/commit/66013d1d64627216d74ce60df42e63e203ca7c35

Once the library user's actor prepares snapshot data, it gets handed over to some snapshot store - this is currently "only compacting the log", but it's pluggable and could use some other persistent datastore. :-)