Photos by Chuck France/Special to The Star Somewhere in the middle of a show that lasted two and a half hours (intermission included) and comprised two dozen songs, David Crosby reminded his audience: This is our 40th year together. By "our" he meant Crosby, Stills and Nash. But the trio's career goes back farther than that and it includes membership in some of the more elite bands in rock history (the Byrds, Buffalo Springfield, the Hollies).