It's possible to find out from WorkerMetrics which workers are not
actively polling for tasks.  However it's not possible to tell if
non-polling workers are merely idle (parked) or if they are stuck.
By adding the worker id (same usize as used in WorkerMetrics calls)
to the on_thread_park() and on_thread_unpark() callbacks it is
possible to track which specific workers are parked.  Then any
worker that is not polling tasks and is not parked is a worker
that is stuck.

With this information it's possible to create a watchdog that alerts
or kills the process if a worker is stuck for too long.