任务处理器:一个慢速操作员任务冻结其应用程序的每组

`ProcessorActions::execute_tasks` (`linera-base/src/task_processor.rs`) states: "The outcomes of distinct groups commute: each is submitted as soon as its task succeeds, in no guaranteed order relative to the other groups, and a task that fails is retried without holding the other groups back." This holds for a group that fails quickly. It does not hold for one that is slow or hung.