(Reuters) - Traders bet on Friday that the Federal Reserve will start easing policy in September with a big half-percentage-point interest rate cut, after government data showed employers added far fewer workers than expected last month, and the jobless rate rose.    Interest rate futures contracts now reflect about a 70% chance seen of a half-percentage-point rate cut next month, versus what was seen before the report as a 70% chance of a more usual quarter-point cut.