Don't make too many circuits once we're separating streams by domain

In legacy/trac#3455 (moved) we're heading toward a world where each Torbutton user has a separate circuit for each referer header. So rather than using the same circuit for an up-to-10-minute period, we could potentially be asking for way way more circuits. That's most bad because of the CPU load it will place on relays.

How many circuits is that exactly, for various client behaviors? We should instrument Tor clients to track how many circuits we would have made under various proposal-171 variations, to get a better handle on how concerned we should be.

Should we consider extending the 10-minute expiry time, to balance the growth in circuits?

Right now we abandon a circuit as soon as any stream takes 10 (or 15) seconds to hear its begin cell. How many circuits are we throwing away early in practice? That's another opportunity to reduce the pain here.