Writes a notification for a given user.

def WriteUserNotification(self, notification):
    """Writes a notification for a given user."""
    if notification.username not in self.users:
      raise db.UnknownGRRUserError(notification.username)

    cloned_notification = notification.Copy()
    if not cloned_notification.timestamp:
      cloned_notification.timestamp = rdfvalue.RDFDatetime.Now()

    self.notifications_by_username.setdefault(cloned_notification.username,
                                              []).append(cloned_notification)