tor/changes/microdesc_use

  o Major features
    - Clients can now use microdescriptors instead of regular descriptors
      to build circuits.  Microdescriptors are authority-generated and
      -authenticated summaries of regular descriptors' contents, designed
      to change very rarely.  This feature is designed to save bandwidth,
      especially for clients on slow internet connections.  It's off
      by default for now, since nearly no caches support it, but it will
      be on-by-default for clients in a future version.  You can use the
      UseMicrodescriptors option to turn it on.