tor/changes/feature15775-fallback

  o Major features (directory mirrors):
    - Include an opt-in trial list of Default Fallback Directories in
      add_default_fallback_dir_servers().
      "Tor has included a feature to fetch the initial consensus from nodes
       other than the authorities for a while now. We just haven't shipped a
       list of alternate locations for clients to go to yet.
       Reasons why we might want to ship tor with a list of additional places
       where clients can find the consensus is that it makes authority
       reachability and BW less important.
       We want them to have been around and using their current key, address,
       and port for a while now (120 days), and have been running, a guard,
       and a v2 directory mirror for most of that time."
      We exclude BadExits and tor versions that aren't recommended.
      We include an IPv6 address for each FallbackDir (#8374).
      (Tor might not use IPv6 fallbacks until #6027 is merged.)
      The unit test ensures that we successfully load all included
      default fallback directories.
      Closes ticket #15775. Patch by "teor".
      OnionOO script by "weasel", "teor", "gsathya", and "karsten".