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 
I've made this change (64d9bca), so the dummy entry at the beginning of the log is never returned as an Entry, only as its Term. It turned out to be a farther-reaching change than I thought, though, since it changes assumptions made in a lot of tests (and ends up reversing some changes we've made recently to make entry zero more like a normal entry).