tor-proto: Make ClientCirc::allow_stream_requests take a HopNum.
For consistency with the other ClientCirc
APIs,
ClientCirc::allow_stream_requests
now takes a HopNum
argument. Upon
receiving an incoming stream request, the reactor now checks if the
request came from the hop specified in allow_stream_requests
(and if
it came from a different hop, the circuit is closed).
Closes #1009 (closed)