It’s been quite a year for Sarah Koenig. The Sagaponack native and Sag Harbor summer resident has gone from being known among a small, quirky subset of brainy public radio listeners for “This American Life” to what might be called an international sensation among a larger, quirky set of brainy podcast listeners. All for “Serial,” which has set iTunes records for being the fastest podcast to reach more than 5 million downloads and streams.