annotate torperf output with paths

Torperf's results right now represent the average performance that users should expect to see. But part of the problem with the Tor network right now is that the variance is high. One component contributing to this variance is the fact that different users choose different entry guards. It seems clear that Tor network performance is a function of (among other things) the entry guards you've picked.

There's lots of analysis we should do to try to learn more about what performance you should expect from having which guards. A prerequisite for all of this analysis is knowing which first hop is used in each torperf run.

So we should modify torperf to annotate in its logs what path was chosen for that request. (For now, we only really want the first hop; but I bet we'll find a good use for knowing the other hops too.)