code: func (c *Client) EstimateFeeAsync(numBlocks int64) FutureEstimateFeeResult {
  	cmd := btcjson.NewEstimateFeeCmd(numBlocks)
  	return c.sendCmd(cmd)
  }
docs: // EstimateFeeAsync returns an instance of a type that can be used to get the result
  // of the RPC at some future time by invoking the Receive function on the
  // returned instance.
  //
  // See EstimateFee for the blocking version and more details.