The Third Bullet by Stephen Hunter Simon & Shuster, $26.99, 485 pages, ISBN 978-1-4516-4020-5 This November twenty-second will mark the fiftieth anniversary of the assassination of John F. Kennedy, the thirty-fifth president of the United   States.  For those of us who are old enough to remember that horrific day and its aftermath, it hardly seems possible that so much time has passed.  In the words of a popular