Receives a single message through Fleetspeak.

def _ReceiveOp(self):
    """Receives a single message through Fleetspeak."""
    try:
      fs_msg, received_bytes = self._fs.Recv()
    except (IOError, struct.error):
      logging.critical("Broken local Fleetspeak connection (read end).")
      raise

    received_type = fs_msg.data.TypeName()
    if not received_type.endswith("grr.GrrMessage"):
      raise ValueError(
          "Unexpected proto type received through Fleetspeak: %r; expected "
          "grr.GrrMessage." % received_type)

    stats_collector_instance.Get().IncrementCounter("grr_client_received_bytes",
                                                    received_bytes)

    grr_msg = rdf_flows.GrrMessage.FromSerializedString(fs_msg.data.value)
    # Authentication is ensured by Fleetspeak.
    grr_msg.auth_state = jobs_pb2.GrrMessage.AUTHENTICATED

    self._threads["Worker"].QueueMessages([grr_msg])