* Benchmark index rises most in six months in January * Market readies for key jobs data due Friday * Grains knocked lower by profit taking * Energy markets dominated by spread trading NEW YORK, Jan 31 (Reuters) - Commodities handed back some of the previous day's gains on Thursday, but stronger crude oil, corn and cotton prices in January pushed the complex to its best monthly performance in six months in January. Soybeans and wheat were pushed lower on Thursday by profit t