Rising oil and gas prices have brought big oil, plenty of workers and lots of housing headaches to the nation’s fastest-growing boomtowns. The small city of Williston, N.D., was once a sleepy farm town — until oil companies discovered ways to tap the vast Bakken formation believed to hold as many as 24 billion barrels of oil. “It’s a game-changer, a bonanza,” said Tom Rolfstad, executive director for the city economic development department.