By Tom Polansek, P.J. Huffstutter and Karl Plume CHICAGO (Reuters) -U.S. data vital to global grain and soybean trading has gone dark during the federal government's shutdown, leaving commodity traders and farmers without crop production estimates, export sales data and market reports during the peak of the autumn harvest. The data blackout comes at a particularly difficult time for farmers, who are grappling with low grain prices and uncertainty over damage to corn and soy fields from dry weather and cr ...