Try clearing stream-isolation state on newly opened rendezvous circs
When a newly created general-purpose circuit is opened, we try to attach streams to it, and if we can't, we clear the circuit's stream-isolation state and try attaching streams again. We tried to do that for rendezvous circuits, too, but at the wrong time (when a rendezvous point was established), and it broke newly created rend circs.
We should try clearing a rend circ's stream-isolation state when (a) it enters ‘C_REND_JOINED’ state and (b) we don't have any streams to attach to it.