Thresholds of nodes to build circuits should be tunable and maybe consider weights too

On legacy/trac#5343 (moved), Mike notes that he doesn't like the 1/3 threshold for having sufficient exit nodes to build circuits.

Arguably, this threshold (and the other threshold changed in legacy/trac#3196 (moved)) should be consensus parameters too.

Arguably, there should also (or instead?) be a minimum threshold by weight.

I'm marking this for 0.2.4.x, but it's likely to be very backportable.