USA’s new series “Unsolved: The Murders of Tupac and the Notorious B.I.G.,” focuses on the failed investigations into the murders of both hip-hop stars, killed when they were in their 20s and at the height of their fame. Was it a function of the so-called East Coast-West Coast rap wars at the time? Was it a gang hit? And were rogue cops from the Lapd involved? Plenty of theories have been bandied about through the years, and “Unsolved” dives into those details with new gusto.