U.S. markets and stock ETFs retreated Friday, stumbling after a strong week, on concerns over global growth in response to poor data out of China. On Friday, the Invesco QQQ Trust (NASDAQ: QQQ) was down 0.9%, SPDR Dow Jones Industrial Average ETF (NYSEArca: DIA) fell 0.7%, and SPDR S&P 500 ETF...