o Minor feature (HS popularity countermeasure):
    - To avoid leaking HS popularity, don't cycle the introduction point
      when we've handled a fixed number of INTRODUCE2 cells but instead
      cycle it when a random value of introductions is reached thus making
      it more difficult for an attacker to find out the amount of clients
      that has passed through the introduction point for a specific HS.
      Closes ticket 15745.