An Americanized version of British television's "Upstairs, Downstairs,"Beacon Hill" will traffic in the same mix of class distinction, historical detail and backstairs hankypanky. The series has two layers--rich family on top, servants beneath. This is a Boston family, the Lassiters--Ben, a powerful industrialist and political boss, and his wife, children and grandchildren. Below stairs, we find Mr. and Mrs. Hacker running the household staff of Irish immigrants--most prominently Brian Mallory, a brash laddie who is determined to rise in the world in a hurry, just as Ben Lassiter did a generation earlier.