o Minor bugfixes (onion service v2):
    - When sending the INTRO cell for a v2 Onion Service, look at the failure
      cache alongside timeout values to check if the intro point is marked
      as failed. Previously, we only looked at if the relay timeout values.
      Fixes bug 25568; bugfix on 0.2.7.3-rc. Patch by Neel Chauhan.