Defend against path bias and tagging attacks

In https://lists.torproject.org/pipermail/tor-dev/2012-March/003347.html, some dude who claims to be a raccoon proved that tagging attacks are an amplification attack that allow an adversary who has c/n of the network bandwidth to compromise of c/n of all circuits through the network.

The tagging attack he describes is actually a subset of path bias attacks we've known about for a long time. Tagging is just a particularly nasty one that also allows for a level of amplification that we previously were not aware of.

This ticket is to serve as the parent ticket for several things we can do to improve the situation and defend against tagging in specific or path bias in general.