The Boston Symphony's youthful, brilliant new music director, Andris Nelsons, leads three programs with his re-energized orchestra. On April 15, there's Mozart's Piano Concerto No. 27 (with Richard Goode), the local premiere of Gunther Schuller's "Dreamscape," and a Nelsons party piece, Strauss's "Ein Heldenleben." April 16 sees two works by Shostakovich, including the Symphony No. 10, plus Christian Tetzlaff in Beethoven's Violin Concerto. But on April 17there's just one work: Mahler's mighty, tragic Symphony No. 6.