Assert that HS-related operations are not performed using single-hop circuits
When I tested my initial try at implementing #2553 (moved), an assertion failed when Tor tried to attach a stream to a single-hop circuit, and appropriately so -- that is almost always a Bad Thing for Tor to do.
But the fact that Tor got that far means that Tor doesn't assert that HS-related directory operations, introduction-point operations, and rendezvous-point operations are not performed over single-hop circuits. It probably should.