“The Diplomat,” starring Keri Russell, returns for a second season on Netflix on Thursday. But maybe the real headline is: A streaming original you’ve long forgotten about is back. If that seems petty, you could argue the pettiness emanates from media companies that do little in their power to get shows back in front of audiences in a reasonable time frame. Network shows manage to do that within the same calendar year. “The Diplomat”? A year and a half. But who’s counting? I liked what creator Debora Cahn came up with for the first season, drawing on her past experiences writing on “Homeland” …