A man authorities had long suspected of a 1989 Dorchester homicide admitted to the killing in court Tuesday, exactly one year after his murder trial for the crime ended in a hung jury. Eugene Sutton, 48, pleaded guilty Tuesday to manslaughter in the fatal stabbing of Richard Gleason, 38, of Cambridge, prosecutors said. Sutton was sentenced to 19 to 20 years in prison, the maximum for the offense. Jury selection for Sutton’s retrial on the murder charge was scheduled to begin Wednesday before the prosecution and defense agreed on a plea deal. Prosecutors say Sutton demanded money from Gleason, then stabbed him to death in the kitchen of a School Street apartment on May 16, 1989.Nicholas Jacques can be reached at firstname.lastname@example.org.