BOSTON — Keith Lockhart was just 35 and a musical wunderkind when he took over as conductor of the Boston Pops in 1995.Now in his 20th season, Lockhart has raised his baton 1,700 times to lead the group nicknamed "America's Orchestra." Along the way, he's shared the stage with more than 250 guest artists as diverse as Aerosmith, Robert DeNiro and Big Bird.