Stop assuming that Tor will download a consensus flavour that contains exit policies

Created by: teor2345

We spend a lot of time messing with tor options to make sure it has a consensus flavour that contains an exit policy. Instead, let's just look for the exit policy in the consensus, and if it isn't there, in the microdescriptors.

This will save us from a whole lot of future bugs where people deploy with the wrong torrc options.

For details, see: https://trac.torproject.org/projects/tor/ticket/25979#comment:4