Jayson Tatum knows his surprisingly limited Olympic role will give him extra motivation when the Celtics’ season begins this fall. But that didn’t make it any more palatable in the moment. After Team USA defeated France 98-87 on Saturday to claim its fifth straight gold medal, Tatum acknowledged that his lack of playing time was difficult to digest. Tatum, a first-team All-NBA selection in each of the last three seasons, played in just four of t…