Brian Steinberg Senior TV EditorMSNBC die-hards tune in partially because they know what they can expect on most evenings: Rachel Maddow will do her 22 minutes of connecting-the-dots news-cycle analysis at 9 p.m., and Brian Williams will wrap up the day with a vast array of knowledgeable experts at 11.Starting next year, however, fans — and even the executives who run the place — can’t be sure what they’re going to get.Williams announced Tuesday that he will walk away from MSNBC and NBC News