By Chris Mannix The Finals loss on the parquet floor still stings. The trade rumors may linger. And a controversy that forced a coaching change hangs over the franchise. But, after a long offseason, the Celtics, with a stacked roster and loads of motivation, are focused on moving forward and bringing another title to Boston. In the underbelly of TD Garden, beneath the black upholstered seats and a long outlet pass from the famed parquet floor, the home and road locker rooms lie parallel to each other, separated by only a narrow hallway and a white paint-splashed concrete wall. A very un-soundp…