Refactor storage package's raft interface.

It's still using a dummy implementation, but it no longer relies on
sending channels and mutable reply buffers through raft.

@spencerkimball @cockroachdb/developers 
Currently no: the raft package transparently forwards commands proposed on a follower to the leader and that makes it very difficult to figure out what's going on with certainty. 

If we disable this automatic forwarding and only proposed commands on the leader, then I think we could do this by recording the current raft log position when we call Propose, and watching for log truncations that remove that record.