sendUserNotification(room, notification) {
    return this.send("UserNotification", {
      action: notification.action,
      collection: notification.collection,
      controller: notification.controller,
      index: notification.index,
      protocol: notification.protocol,
      result: JSON.stringify(notification.result),
      room,
      status: notification.status,
      timestamp: notification.timestamp,
      user: notification.user,
      volatile: JSON.stringify(notification.volatile),
    });
  }
Publishes an event telling that a user notification must be
propagated

@param  {string} room - Koncorde room
@param  {UserNotification} notification
@return {Long} Id of the message sent to other nodes