Archiving b/c we discovered jest.getTimerCount() which is good enough for now.

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Provides a minimalistic queue for in-flight operations.

This allows tests to change their behavior from "wait until bespoke condition Y is true" to "wait until outstanding operations are done".

This is a standalone package with no dependencies so that it can be called from production code.

In theory this could be useful for driving a mini-dashboard/spinner of "operations in-flight" or "failed async operations", but really the core reason is to provide a dependency that doesn't need to be mocked out.