Finish 'encrypted services' proposal

Many hidden services use the hidden service design for its incidental properties, such as end-to-end encryption, self-authenticating names, and usability properties of knowing you're going through Tor. They suffer from the performance and robustness issues of hidden services when (maybe) they don't need to.

We should offer a way to run a Tor hidden service where the server-side rendezvous circuits are just one hop.

Step one is to finish the proposal.