Bug: `BufferedActions` hanging action

[heyltsjay]

BufferedActions will queue, but not handle, actions that come in while it's in the process of sending. This results in the action dangling in the buffer indefinitely until some other action comes through. This is a regression from 0.19.0

To Reproduce
Buffer Bug.zip

1. Run the example project
2. Press the button on ViewController
3. Note that viewStore.send(.didLoad) is executed as expected in ViewControllerB
4. Note that .didLoad is not handled by either reducer
5. Dismiss ViewControllerB
6. Note that .didLoad and didDismiss are handled by reducer in succession.

Expected behavior
.didLoad should be processed by the reducer on the runloop it was sent (or at least in a timely manner)

Environment

• swift-composable-architecture version >= 0.20.0 (not an issue in 0.19.0)
• Xcode 12.5

[mbrandonw]

This must be a consequence of #619 where we combined the notions of "synchronous actions" and "buffered actions". We thought it was logically equivalent, but perhaps not :/

Would you be able to get us a failing test so that we can look into a fix?

[heyltsjay]

Sure, added in #662

[stephencelis]

Fixed in #662, thanks!