JUBA, South Sudan — In July 1995 Nedzad Handzic survived the Srebrenica massacre. More than 100 of his relatives, friends and neighbors did not. When Serbian troops descended on the Bosnian city, UN peacekeepers who had declared it a safe haven stood by as 8,000 people were killed. It was a dark, shameful episode in peacekeeping’s short history, and one the UN has endeavored not to repeat. But this past December, Handzic felt like it was happening all over — in another conflict in another city on another continent.