Add (add,remove)_from_rcv3 functions

This is (temporarily) in a separate module. It can probably stay there, or it could be moved; either way, I'm doing it that way right now, because there's a lot of open PRs, and this essentially reduces the change of merge conflicts to zero.

This does *not* yet actually hook up these functions to e.g. `otter.worker.launch_server_v1.(add_to,remove_from)_load_balancer(s?)`, because small PRs are good :)
Oops, nm, I think it doesn't, since it looks like the `get_request_func` calls `add_bind_root`, which [uses `append_segments`](https://github.com/rackerlabs/otter/blob/master/otter/util/pure_http.py#L180)