Traders are betting on fresh upheaval in the oil market, in an abrupt turnaround from the calm of recent months. U.S. oil prices have slid 12% since the end of August, and on Tuesday notched their biggest one-day drop since June. Though the market has clawed back some of those declines, many say trading in the options market shows investors preparing for further drops, as pressures on crude prices mount from forces including a slow recovery in fuel demand and the presidential election. …