Like it or not, millions of Americans are giving up Downton Abbey for Lent. Or at Lent. The final episode aired Monday. “Keep calm and wait for September” the Facebook icons reassure. You wouldn’t expect to find such a cool set-must see TV-water cooler buzz-show on PBS, but Masterpiece Theater has struck gold (four Emmys) with its story of Lord and Lady Grantham, their three Jane Austinesque daughters and the downstairs intrigue of butlers, valets, footmen, ladies maids and cooks. What hooks millions of viewers all over the world? A world of daily candlelight dinners, beaded Chanel gowns, side-saddle hunts and Christmas pheasant shoots is rocked by World War I,…