Write tool to automate web queries to Tor; and use Stem to track stream/circ allocation and results

As part of #5752 (moved) we need to know how many circuits we're making now, how many we're discarding early because a stream didn't work, etc.

This is a two-part project: first is a tool to automatically make a series of requests to Tor, in a repeatable way, and second is a Tor controller script, probably using Stem, that watches stream and circuit events (and maybe more), and tracks which streams get allocated to which circuits, how many total circuits are made, how quickly results return, and other statistics. Then we would change the underlying Tor, replay the same set of requests, and know what circuit behaviors to expect.

I expect we'll also discover that we don't export enough info via the control protocol to make good conclusions; in that case we'll also want to modify Tor to export this info.