THESE HIGH, GREEN HILLS In These High, Green Hills we're once again in Mitford, a southern village of local characters so heartwarming and hilarious you'll wish you lived right next door.    At last, Mitford's rector and lifelong bachelor, Father Tim, has married his talented and vivacious  neighbor, Cynthia. Now, of course, they must face love's challenges: new sleeping arrangements for Father Tim's sofa-sized dog, Cynthia's urge to decorate the rectory Italian-villa-style, and the growing pains of the thrown-away boy who's become like a son to the rector.    Add a life-changing camping trip, the arrival of the town's first policewoman, and a new computer that requires the patience of a saint, and you know you're in for another engrossing visit to Mitford—the little town that readers everywhere love to call home.