WASHINGTON (Reuters) - The U.S. Senate on Thursday advanced a compromise gun control plan to stop people on some terrorism watch lists from buying guns, in the wake of the June 12th mass shooting in Orlando, Florida. While the measure, spearheaded by Susan Collins, a Republican from Maine, cleared one procedural hurdle, it was not clear if the Senate would move further ahead on it. Passing the bill in the House of Representatives could also be an uphill battle.  