WIP: (Experiment) Mutex based synchronous queue

[BalmungSan]

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At this point, I am sure this will not be merged because of some limitations of the change, but I thought it was worth having it here for future reference.

The implementation has a problem: It requires Concurrent instead of GenConcurrent which I am sure breaks MiMa.

Also, we also should benchmark to ensure this version is not considerably slower than the current one. Plus, it is no longer way simpler than the original one.

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Anyways, the two first commits of this branch are probably worth merging. The first one may be added to Daniel's #3606 And I may create another PR for the second one.