NEW YORK (Reuters) - A protracted trade war between China and the United States, the world's largest economies, and a deteriorating global growth outlook has left investors apprehensive about the end to the longest expansion in American history.The recent rise in U.S.-China trade war tensions has brought forward the next U.S. recession, according to a majority of economists polled by Reuters who now expect the Federal Reserve to cut rates again in September and once more next year.Trade tensions..