It’s cold, still dark early, a time, as the cliché has it, to curl up with a good book. And I’ve got one for you, if “good” means almost non-stop reading because you care about the main characters, even if they’re not good. And they’re not, in Anthony Schneider’s new novel, “Lowdown.” They’re Mafia, but as “The Godfather” and Tony Soprano proved, complex goodfellas can fascinate. In “Lowdown” Schneider delivers an absorbing tale about a guy whose crime family has real-life connections to the