metrics consensus health should check the consensus from each authority

While doing a training yesterday, my poor os x user started tbb and it fetched a consensus that had two signatures on it -- one from gabelmoo-legacy, and one from moria1-legacy.

I didn't catch which authority gave it to her.

Several minutes later, Tor tried another authority and bootstrapped from there.

While we're checking network health, we should fetch a consensus from each authority and see if it is recent enough and has enough signatures on it.