The default TaskScheduler may also mark tasks as long running after a
while and then spawn an extra thread regardless. But this way we make
sure to not block the threadpool by spawning the tasks in separate
threads instead in the normal pool.
Also keep the denychildattach which Run would have added. See:
https://devblogs.microsoft.com/pfxteam/task-run-vs-task-factory-startnew/
