tor/changes/ticket19769

  o Major features (security):
    - Change the algorithm used to decide DNS TTLs on client and server side,
      to better resist DNS-based correlation attacks like the DefecTor attack
      of Greschbach, Pulls, Roberts, Winter, and Feamster).  Now
      relays only return one of two possible DNS TTL values, and clients
      are willing to believe DNS TTL values up to 3 hours long.
      Closes ticket 19769.