Broadway Musicals Are Nation’s Operas, Glimmerglass Staple By LIBBY CUDMORE • Special to www.AllOTSEGO.com COOPERSTOWN – Since Francesca Zambello became general & music director at the Glimmerglass Festival in 2011, one show has been in the back of her mind. “I’ve always wanted to do ‘Show Boat’,” she said. “It’s one that I’ve always had on our docket.” The Oscar Hammerstein-penned musical, which deals with family and racism in post-Reconstruction Mississippi, is set to open the festival season this Saturday,…