RCv3 requests

Early work in progress!

This probably should wait until the following PRs are merged:

- New dependent effect features: #730, #732, #733
- `rcv3-steps`: #728 

Once that's in master, I'll merge master back into this, and hopefully the diff will look a little less gnarly.

Once this PR and #734 is done, we can tie the RCv3 stuff together in `otter.worker.launch_server_v1` and then we're just a test away :)