Here he goes again on his own. Going down the only road he’s ever known. OK, David Coverdale isn’t totally on his own. The 57-year-old singer has a new Whitesnake lineup behind him. But he is going down the only road he’s ever known on an old-fashioned, heavy-metal double bill: Whitesnake and Judas Priest play the Comcast Center on Tuesday. As Coverdale pointed out in a phone interview from his Lake Tahoe home, “You’re not going to listen to Whitesnake for environmental songs or to change your life. You’re going to listen to Whitesnake for hard rock.”