By Lenin Ocampo and Uriel Sanchez SAN MIGUEL TOTOLAPAN, Mexico (Reuters) – A fight between two rival gangs in the violence-plagued southwestern state of Guerrero left 20 dead, including a local mayor, and two more wounded, Mexican authorities said Thursday. Among those shot dead in San Miguel Totolapan’s town hall on Wednesday were Mayor Conrado Mendoza, his father and former Mayor Juan Mendoza, and other local officials, according to a statement from state authorities. A minor was also among those killed, authorities said. Photos and videos shared on social media show the town hall riddled wi…