Arthur McKeach and Nick Cistero have been behind most of the loan-sharking, extortion, hijacking, illegal gambling, union corruption and drug-dealing in Greater Boston for almost four decades - and every cop in the Boston Police Department knows it. So what's kept them on the streets for so long? What the cops don't know is that McKeach and Cistero have a sweet deal going with the Boston office of the FBI: the bureau looks the other way when they're doing business, and even gives them a heads-up when the local or state cops start.