CLOSE The NFL’s 2020 free agency period could trigger quite a game of musical chairs among several of the league’s quarterbacks, some established stars and others at least noteworthy. Tom Brady, Drew Brees, Teddy Bridgewater, Case Keenum, Eli Manning, Marcus Mariota, Dak Prescott, Philip Rivers, Ryan Tannehill and Jameis Winston all have expiring contracts. It’s hard to imagine…