Some day soon, a 50KB bandwidth test will be too low for the Fast flag

Right now the bandwidth self-test involves sending 125KB over each of 4 circuits. Being able to both send and receive it within 10 seconds earns you a self-measured bandwidth rating of 50KB/s.

That's supposed to be enough to bootstrap you into the network, and from there a few clients will pick you, you will realize you're faster than 50KB/s, and away you go.

The threshold for the Fast flag right now is 40KB. What if it went up to 51KB, due to natural network growth or due to steps like legacy/trac#1854 (moved)? Then we might as well not do the bandwidth self-test at all. (At that point we rely on the bwauths to push traffic over us for us to notice that we can push traffic; and since the bwauths choose how much to send based on how much you self-report, that might not be reliably enough either.)

I think we should raise it to "over 8 circuits" rather than "over 4 circuits", i.e., test twice as many bytes.