Build and test most of the machinery needed for IPv6 virtualaddrmaps
With an IPv6 virtual address map, we can basically hand out a new IPv6 address for _every_ address we connect to. That'll be cool, and will let us maybe get around prop205 issues. This uses some fancy logic to try to make the code paths in the ipv4 and the ipv6 case as close as possible, and moves to randomly generated addresses so we don't need to maintain those stupid counters that will collide if Tor restarts but apps don't. Also has some XXXX items to fix to make this useful. More design needed.