A Supreme Court justice, a defense secretary and an FBI director walk into a bar …If that sounds like the beginning of a joke, you’re close. It was the cold open for “Saturday Night Live” over the weekend, and the sketch wasn’t set in some generic D.C. watering hole but an actual local landmark: Martin’s Tavern, the Georgetown institution that has served presidents and politicos continuously since it opened in 1933, shortly after Prohibition was repealed.