Raft cleanup

(This diff will get easier to read once #198 is merged; you only need to look at the last two commits in this PR)

```
commit 971b50efb72f5bda95137c1126071672533225b2
Author: Ben Darnell <ben@bendarnell.com>
Date:   Mon Dec 8 17:28:24 2014 -0500

    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.

commit e982b0487f14fc75f41e5d773a369354dd3f537b
Author: Ben Darnell <ben@bendarnell.com>
Date:   Mon Dec 8 15:42:44 2014 -0500

    Remove unused cruft from multiraft.
    
    Just expose the underlying raftpb types directly instead of creating our
    own wrappers.
    
    Also clean up some obsolete todos.
```

@spencerkimball @cockroachdb/developers 
Is your thought here that LoadGroups() for the non-memory case would actually just return the store's ranges map and have Range implement WriteableGroupStorage?