Raft: log storage interface and multinode integration

This branch introduces a pluggable interface for on-disk storage of logs, and brings MultiNode up to date with some upstream changes in the Node interface (the new Advance method). It's still something of a work in progress, but the next step is to implement this interface in cockroach to make sure it meets our needs before pushing it back upstream.

@spencerkimball @cockroachdb/developers 
If i+1 is greater than l.unstable, are we supposed to truncate the prefix of l.unstableEnts?