The deli worker accused of fatally stabbing his Harlem ex-girlfriend in a jealous rage early Monday was coolly going about his business at his Bronx job just hours before the slaying, haunting new video shows. Luis Marquez , 41, is facing murder charges for allegedly knifing Carmen Lopez up to 50 times when he ambushed her inside her West 129th Street apartment building around 4 a.m. after lying in wait for two hours. But leading up to the frenzied attack, footage obtained by The Post showed