New York socialite and author Jean Shafiroff and her husband, investment adviser Martin Shafiroff, have bought a stunning, modern, waterfront gated home in Manalapan, Fla. — the exclusive billionaires’ enclave between Delray and Palm Beach — for around $21 million in an off-market sale, Gimme Shelter can reveal. Inside the Manalapan mansion. Past and present…