[for python 3.9] cancel_futures on all ThreadPoolExecutors

[benclifford]

Is your feature request related to a problem? Please describe.

When the minimum Python version moves from Python 3.8 to 3.9, the cancel_futures parameter to concurrent.futures.ThreadPoolExecutor will become available. That should be added onto all ThreadPoolExecutor shutdowns, which should make Parsl shutdown happen faster/with less stuff happening.