bwauths learn to recognize Unmeasured=1 in consensus line and treat it differently

In #2286 (moved) we're going to change directory authorities to say something like "w Bandwidth=100 Capped=1" for relays that don't have enough Measured lines.

Torflow's BWAuthority needs to learn not to get snookered by the 100 into thinking that the relay's peers are all the other "Bandwidth=100" relays.