o Major bug fixes (circuit building):
    - Tor authorities, relays, and clients no longer support
      circuit-building using TAP. (The hidden service protocol
      still uses TAP.)
    - Relays make sure their own descriptor has an ntor key.
    - Authorites no longer trust the version a relay claims (if any),
      instead, they check specifically for an ntor key.
    - Clients avoid downloading a descriptor if the relay version is
      too old to support ntor.
    - Client code ignores 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.
      Fixes bug 19163; bugfix on 0.2.4.18-rc.