Barry Hammer didn’t know what hit him. A night out at a Celtics game with his wife and twin boys on Tuesday turned into a nightmare when he was punched in the face by a total stranger for no apparent reason. “I’ve never been punched in my life,” said the 38-year-old Auburn dad who’s still stunned by what happened in the mezzanine area of Boston’s TD Garden during his kids’ first Celtics game. Hammer said he believes he was likely the victim of a so-called “knockout game.” The game has been seen around the country and involves attackers who sucker...