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 :)
The RCv3-specific support is in #759. I'll merge #759 into this branch, and double-check the success codes.