tor/changes/fallbacks-201604

  o Minor features (fallback directory mirrors):
    - Give each fallback the same weight for client selection;
      restrict fallbacks to one per operator;
      report fallback directory detail changes when rebuilding list;
      add new fallback directory mirrors to the whitelist;
      update fallback directories based on the latest OnionOO data;
      and any other minor simplifications and fixes.
      Closes tasks 17158, 17905, 18749, bug 18689, and fixes part of
      bug 18812 on 0.2.8.1-alpha; patch by "teor".