Remove unused cruft from multiraft.

Just expose the underlying raftpb types directly instead of creating our
own wrappers.

Also clean up some obsolete todos.
Streamline Multiraft.start select loop.

raft.MultiNode is thread-safe, so we no longer need to route everything
through our own goroutine.

Remove some unneeded generality in the transport interface.
Merge pull request #204 from bdarnell/raft-cleanup

Raft cleanup