Loading minitrees with num_workers can lead to undying processes

[JelleAalbers]

When you load minitrees with multiple processes using the num_workers option, but the load crashes or is interrupted, it seems to be possible that some process remain alive even after restarting your notebook kernel. This might be one of the reasons many people have a lot of processes open on the jupyterhub.

I'm not sure what we can do against this, except perhaps investigate and try to isolate the issue. If we have a clear example we can report it upstream to dask or jupyterhub (wherever the problem seems to lie).