Leftover resource_tracker processes

[Min RK]

I'm still not quite sure where these come from (multiprocessing for sure, but exactly where is unclear), but sometimes when starting and stopping lots of clusters, there can be quite a few processes leftover after the cluster is shutdown, like:

python -c from multiprocessing.resource_tracker import main;main(26)

When I try to track it down, I can't seem to provoke their creation on purpose, but something isn't being cleaned up right.