David Crosby is a Rock and Roll Hall of Famer twice over: inducted in 1991 for his work as a founding member of The Byrds and again six years later for the folk-rock supergroup Crosby, Stills and Nash. But the 77-year-old isn’t stuck in the past. He’s collaborating with a new circle of younger musicians and has released four albums in five years, with another on the way. He performs May 15 in St. Louis at the Sheldon Concert Hall.