o Major features (directory authority):
    - Authorities can now be configured to label relays as "MiddleOnly".
      When voting for this flag, authorities automatically vote against
      Exit, Guard, HSDir, and V2Dir; and in favor of BadExit.
      Implements part of proposal 335. Based on a patch from Neel
      Chauhan.
    - Add a new consensus method to handle MiddleOnly specially. When
      enough authorities are using this method, then any relay
      tagged with the MiddleOnly flag will have its Exit, Guard, HSDir,
      and V2Dir flags automatically cleared, and will have its BadExit flag
      automatically set. Implements part of proposal 335.