o Major bug fixes (circuit building):
    - Tor authorities, relays, and clients only use ntor, except for
      rare cases in the hidden service protocol.
    - Authorities, relays and clients specifically check that each
      descriptor has an ntor key.
    - Clients avoid downloading a descriptor if the relay version is
      too old to support ntor.
    - Client code never chooses nodes without ntor keys: they will not
      be selected during circuit-building, or as guards, or as directory
      mirrors, or as introduction or rendezvous points.
    - Circuit-building code assumes that all hops can use ntor,
      except for rare hidden service protocol cases.
    - Hidden service client to intro point and service to rendezvous point
      connections use the TAP key supplied by the protocol.
      Fixes bug 19163; bugfix on 0.2.4.18-rc.