PR #3607 starts a thread at import parsl time

[benclifford]

Describe the bug

Since PR #3607, import parsl starts a thread.

To Reproduce

>>> import threading
>>> threading.enumerate()
[<_MainThread(MainThread, started 140077973014336)>]
>>> import parsl
>>> threading.enumerate()
[<_MainThread(MainThread, started 140077973014336)>, <Thread(Thread-1 (__atexit), started 140077889824448)>]

Expected behavior

Importing parsl should not be doing global environment stuff like starting threads: things like this should start ideally at in .start() methods, or less preferably in config object initializers, and be cleaned up by the relevant shutdown method.

Environment
my laptop, master branch 6ac930e

cc @yadudoc @khk-globus

[benclifford]

I think, although I have not investigated, also starts a new thread in every place that runs parsl code - not only the submitting workflow where a user might explicitly says import parsl, but inside every individual htex worker.