NEW YORK — The body of a slain 22-year-old New York Police Department officer arrived at St. Patrick’s Cathedral for his Thursday wake, with scores of solemn colleagues lining Fifth Avenue in a silent farewell six days after his killing. An NYPD helicopter whirred overhead as Officer Jason Rivera was brought inside the venerable Midtown Manhattan church, with family members arriving prior to the 1 p.m. memorial service open to the public. A lone police bagpiper played “Amazing Grace” as the assembled officers saluted his flag-draped casket on its way inside the cathedral on a brisk January day…