NEW YORK (Reuters) - The ebb and flow of trade tensions between the United States and China has roiled financial markets for months, and with the December deadline for higher U.S. tariffs on Chinese goods approaching fast no asset class is immune to trade-related risks.Markets' sensitivity to trade headlines was on stark display this week.The S&P 500 index fell 2% over the first two days of the week as the trade outlook darkened following a U.S. move to impose visa restrictions on Chinese..