The default executor can wrongly indicate that tasks from all task groups are unhealthy

The default executor sets a "global" unhealthy field to true once it kills a task due to failed health checks.

When a task from any task group exits, it will check that "global" healthy field and set the task status update's healthy field accordingly. This means that once an unhealthy task belonging to a task group is killed, task status updates for tasks belonging to other task groups, which can contain only healthy tasks, will be sent with their healthy field set to false.