bugfix in async._kill_task_attempt
bugfix in async._kill_task_attempt

StanfordBioinformatics/loom,StanfordBioinformatics/xppf,StanfordBioinformatics/loom,StanfordBioinformatics/xppf,StanfordBioinformatics/xppf,StanfordBioinformatics/loom,StanfordBioinformatics/xppf,StanfordBioinformatics/loom