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 
Good point. I guess to date we haven't needed a mutex because in the tests we have so far we set up all the listeners in the main thread before doing anything else.