o Minor features (logging, heartbeat):
    - When a relay receives a cell that isn't encrypted properly for
      it, but the relay is the last hop on the circuit, the relay
      now counts how many cells of this kind it receives, on how
      many circuits, and reports this information in the log.
      Previously, we'd log each cell at PROTOCOL_WARN level, which
      is far too verbose to be useful. Fixes part of ticket 40400.