Clear all singleton instances during shutdown

[kiersten-stokes]

Related to #2778

What changes were proposed in this pull request?

This PR clears all singleton variables (those always created during startup) during extension shutdown. Upstream tests that create a ComponentCache instance no longer side-affect their downstream tests that rely on the component cache being in a certain state. This solves the test failures related to the changes made between jupyter-server 1.17.0 and 1.17.1. The larger issue of what change(s) between those versions that now makes this necessary is still in question.

How was this pull request tested?

Tests passing locally even with jupyter-server==1.17.1

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[kevin-bates]

This looks good and makes perfect sense. 1.17.1 has done some app cleanup that must have exposed the fact that we still had some singletons that were not cleared.