Long Island’s most notorious serial killer will help the FBI figure out what makes his sick mind tick — part of his chilling plea deal in eight cold-case slayings, officials said Wednesday. The stipulation that Gilgo Beach serial killer Rex Heuermann, 62, will sit down with the agency’s Behavioral Analysis Unit comes along with a promise that he’ll serve three life sentences without the chance of parole when he goes back in front of the judge on June 17 for sentencing in the gruesome murders of