By Djaffar Sabiti and Fiston Mahamba GOMA, Democratic Republic of Congo (Reuters) -Three United Nations peacekeepers and at least 12 civilians were killed during a second day of violent anti-U.N. protests in eastern Democratic Republic of Congo on Tuesday, authorities said. The protests were spurred by complaints that the U.N. mission, known as MONUSCO, has failed to protect civilians against militia violence which has raged for years. Demonstrations began on Monday in the city of Goma and spread on Tuesday to Butembo, where a U.N. soldier and two U.N. police with the mission were shot dead, U…