tor/changes/bug23826-23828

  o Major features (IPv6, directory documents):
    - Add consensus method 27, which adds IPv6 ORPorts to the microdesc
      consensus. This makes it easier for IPv6 clients to bootstrap and
      choose reachable entry guards.
      Implements 23826.
    - Add consensus method 28, which removes IPv6 ORPorts from
      microdescriptors. Now that there are IPv6 ORPorts in the microdesc
      consensus, they are redundant in microdescs. This change is compatible
      with tor clients on 0.2.8.x and later. (0.2.8.x introduced client IPv6
      bootstrap and guard support.)
      Implements 23828.
    - Expand the documentation for AuthDirHasIPv6Connectivity when it is set
      by different numbers of authorities.
      Fixes 23870 on 0.2.4.1-alpha.