By Eric Onstad LONDON (Reuters) -The London Metal Exchange (LME) was forced to halt nickel trading and cancel trades after prices doubled on Tuesday to more than $100,000 per tonne in a surge sources blamed on short covering by one of the world’s top producers. The LME’s shock move came as Western sanctions threatened supply from major producer Russia and marked the biggest crisis to hit the 145-year-old exchange in decades. In the 1990s a rogue Sumitomo trader tried to corner the copper market and tin trading was stopped for five years in the 1980s. “The current events are unprecedented,” the…