code: func (c *Client) SignMessageAsync(address btcutil.Address, message string) FutureSignMessageResult {
  	addr := address.EncodeAddress()
  	cmd := btcjson.NewSignMessageCmd(addr, message)
  	return c.sendCmd(cmd)
  }
docs: // SignMessageAsync 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 SignMessage for the blocking version and more details.