Liquefied natural gas is fetching the lowest price on record in Asia, a troubling sign for U.S. energy producers who have relied on overseas shipments of shale gas to buoy the sagging domestic market. The main price gauge for liquefied natural gas, or LNG, in Asia fell to $3 per million British thermal units Thursday, down sharply from more than $20 six years ago as U.S. deliveries have swamped markets around the world. As…