tor/changes/bug13814-avoid-exit-paths-no-exits

  o Minor bugfixes:
    - Avoid building exit circuits from a consensus with no exits
      Tor can now build circuits from a consensus with no exits.
      But if it tries to build exit circuits, they fail and flood the logs.
      The circuit types in the Exit Circuits list below will only be
      built if the current consensus has exits. If it doesn't,
      only the Internal Circuits will be built. (This can change
      with each new consensus.)
      Fixes bug #13814, causes fewer path failures due to #13817.

      Exit Circuits:
        Predicted Exit Circuits
        User Traffic Circuits
        Most AP Streams
        Circuits Marked Exit
        Build Timeout Circuits (with exits)

      Internal Circuits:
        Hidden Service Server Circuits
        Hidden Service Client Circuits
        Hidden Service AP Streams
        Hidden Service Intro Point Streams
        Circuits Marked Internal
        Build Timeout Circuits (with no exits)
        Other Circuits?