How will this century sound? Classical music faces a new age in unexpectedly buoyant form. It is livelier and more surprising than at any time since Rachmaninov, says Norman Lebrecht THE final decade of the 20th century brought a measure of relief to the agonised search for a musical identity. Warily, the art looked into a mirror and accepted what it saw, an image fractured and more complex than any in its history. Unlike former centuries which wore a uniform musical face – the 19th century being broadly Romantic, the 18th Classical, the 17th Baroque – the 20th century had…