Don't allow multiple INTRODUCE1s on the same circuit

Currently, it seems like clients are able to send multiple INTRODUCE1 cells to the IP. The result is that many INTRODUCE2 cells reach the HS, which means that the HS will try to establish multiple rendezvous circuits.

This gives a better position to attackers who want to flood a HS with rendezvous circuits (like legacy/trac#15463 (moved)), since with a single circuit they can cause hundreds of rendezvous.

We should fix this in the IP-side, by closing the circuit after sending the INTRODUCE_ACK to the client. An alternate behavior, is to change the state of the circuit after INTRODUCE1 is received and close it if more such cells are received.