use a pair of memory object streams instead of two queues

[graingert]

Summary

Checklist

(discussed on previous PR, no tests needed as this is a plain refactor and existing tests cover it, no doc changes as this is a transparent refactor)

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

Amazing! 🙏