Police, public officials and mourners from across the country gathered Sunday to honor a detective killed last month in an act of violence that has roiled New York City, with a large number of officers again making their frustrations with the city’s mayor apparent by turning their backs on him as he paid tribute to the slain officer. Defying a request from Police Commissioner William Bratton, the strident display of protest began as Mayor Bill de Blasio took the podium at the Aievoli Funeral Home in Brooklyn to eulogize Detective Wenjian Liu, reinforcing entrenched feelings of hostility that have gripped...