o Minor bugfixes (usability):
    - Do not log "circuit_receive_relay_cell failed" at LOG_PROTOCOL_WARN;
      instead log it at DEBUG. In every case where we would want to log
      this as a protocol warning, we are already logging another warning
      from inside circuit_receive_relay_cell. Fixes part of bug 40400;
      bugfix on 0.1.1.9-alpha.