Stop single hop client connections to Single Onion Services

Tor2Web clients make a one-hop connection to HSDirs, intro points, and rend points. Single Onion Services also make a one-hop connection to the rendezvous point.

This uses Tor as a one-hop proxy (in this case, to a single onion service), which we try to avoid, because it enables certain attacks. We also try to avoid single hop connections in the onion service protocol, because they give IP addresses to middle relays.

See the child tickets for details.