By Gram Slattery WASHINGTON (Reuters) – Negotiations in the U.S. Senate to narrow sentencing disparities between crack and powder cocaine have stalled, two sources said on Monday, in what could prove a blow for criminal justice reform advocates. Earlier in December, Reuters reported Senate negotiators reached a tentative deal to narrow the sentencing disparity between the two substances and planned to tuck the measure into a year-end bill funding the government. Mandatory minimum sentences for crack-related offenses are currently 18 times lengthier than those for powder cocaine, which has led …