In the afterglow of “Downton Abbey” and “The King’s Speech,” it would seem like a fortuitous moment to blow the dust off “Upstairs Downstairs,” the beloved British drama about 20th-century Belgravian uppercrusties and their loyal servants — a show that became a PBS “Masterpiece” juggernaut for some 70 episodes and proved deeply satisfying to Viewers Like You until it went off the air in 1977.