Crude oil has become so cheap it could speed the oil market to a recovery. U.S. crude plunged on Thursday to a 12-year low, an ominous milestone for Houston's oil hub, which already has shed thousands of jobs in the 19-month oil downturn. With crude now fetching less than $34 a barrel, about half the nation's scattered collection of 400,000 aging, nearly depleted wells may have to be shut in as their product becomes less valuable than their operating costs. Called stripper wells, these produce a negligible amount of oil individually but together account for about a tenth of U.S....