code: func (sc *Client) NewMessageFilter(ctx context.Context, criteria whisper.Criteria) (string, error) {
  	var id string
  	return id, sc.c.CallContext(ctx, &id, "shh_newMessageFilter", criteria)
  }
docs: // NewMessageFilter creates a filter within the node. This filter can be used to poll
  // for new messages (see FilterMessages) that satisfy the given criteria. A filter can
  // timeout when it was polled for in whisper.filterTimeout.