Modify facilitator to hand out multiple relays.

Currently the facilitator can only give out one relay address to proxies that request clients. Modify it to be able to give out multiple relays, and figure out aspects like load balancing, preferring fast relays, etc.

In the short term (the lifetime of a circuit?), a client needs the same relay. This is because switching relays will change the websocket bridge's fingerprint, which Tor will not tolerate. We need to consider this when a proxy goes down and the client is given a new one.

Trac:
Username: aallai