IN ONE of the most tony towns in America, murder reared it's ugly head in September 1984 and to this day, the case remains colds. Matthew Margolies was just 13 and lived in the Pemberwick area of Greenwich when he was found murdered on Sept. 5, 1984--five days after he vanished. Over the years cops have identified as many as eight suspects in the cold-blooded killing but there have been no arrests and many fear there never will be because of moneyed families that inhabit the area.