John Wall is not yet a “floor general.” He’s not quite a “facilitator,” although he’s closing in on becoming the Washington Wizards’ “engine.” It remains to be seen what sort of metaphor develops to describe the second-year guard out of Kentucky, who visits the TD Garden on Sunday to face the Celtics. More than any other position, point guard lends itself to a range of comparative descriptions. Nearly every one is a sparkplug, a security blanket or a coach — or all three — on the floor.