Improve circuit selection algorithm to rely on more than just bandwidth

[chris-dickson]

Currently, path selection is simulated by bandwidth only. We can improve on this by following the path selection guidelines:

• We do not choose the same router twice for the same path.
• We do not choose any router in the same family as another in the same path. (Two routers are in the same family if each one lists the other in the "family" entries of its descriptor.)
• We do not choose more than one router in a given /16 subnet.
• We don't choose any non-running or non-valid router unless we have been configured to do so. By default, we are configured to allow non-valid routers in "middle" and "rendezvous" positions.
• The first node must be a Guard node.

http://tor.stackexchange.com/questions/113/how-does-a-tor-client-pick-tor-nodes-for-circuit-creation/114#114