Leftover resource_tracker processes

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.

Start with Python's multiprocessing.resource_tracker.main and trace the cluster start and shutdown paths in ipyparallel. Reproduce repeated cluster startup and shutdown, identify where resource_tracker processes remain, and add a regression test showing that shutdown leaves no leftover processes.

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