Don't surprise users with new circuits in the middle of browsing

The way Tor handles circuit dirtiness for websites is pretty crappy. See #13766 (moved).

However, simply raising the circuit dirtiness timeout has a whole bunch of problems. See https://lists.torproject.org/pipermail/tor-dev/2015-March/008548.html.

So instead of raising the timeout, let's make normal circuits behave more like hidden service circuits: keep resetting their timestamp_dirty every time a new stream is attached. This has the effect that a user will never suddenly get a new circuit in the middle of actively using a website, which will be a huge usability improvement. Still not ideal, but good enough to leave the actual circuit dirtiness timeout alone.

I am going to do this with a TBB-specific Tor patch for now so we can test this in TBB 4.5a5. We can then decide if we want to make this a torrc option after that.