Review every use of onehop_tunnel for assumptions that it's a directory connection

Previously, the only onehop_tunnel connections Tor made were for directory fetches.

But now, with Tor2Web and Rendezvous Single Onion Services (and Single Onion Services, when they arrive), several other modes make one-hop connections.

We should check every use of onehop_tunnel to make sure it's not being used as a substitute for checking the purpose / type of the connection.