A suspected serial killer has been arrested over the notorious Gilgo Beach murders in Long Island, The Post can confirm. Rex Heuermann, 59, a married architect at a New York City firm, was caught after DNA from the hair of victim Megan Waterman matched that of his, taken by investigators from a discarded pizza crust in January. The father-of-two — a former classmate of Hollywood actor Billy Baldwin — was arrested Thursday after cops had staked out his home on 1st Avenue in Massapequa Park, Long Island, and office in midtown Manhattan. His arrest is tied to the so-called “Gilgo...