o Minor features (DOS resistance):
    - Count the total number of bytes used storing hidden service descriptors
      against the value of MaxMemInQueues. If we're low on memory, and more
      than 20% of our memory is used holding hidden service descriptors, free
      them until no more than 10% of our memory holds hidden service
      descriptors. Free the least recently fetched descriptors first.
      Resolves ticket 13806.