Add support for routing all storage operations through raft.

The tests pass with this change most of the time but there are some
timing issues around the initial election and tests using IsTrueWithin,
so everything still uses noopRaft until that can be fixed.

@spencerkimball @cockroachdb/developers 
From the looks of this, I would accept the struct to be thread-safe and hence the listeners map to be protected by a Mutex, but it isn't.