DO NOT MERGE Per-queue delivers for old classic queues

[lhoguin]

This is a change that comes from the modern index work and proves beneficial for the old index and could provide a boost in a 3.9 patch release. Context: #3029 (comment)

It's very WIP at the time of opening and the PR is only opened for the sake of tracking the work. Because August will be slow there might not be too much progress at first. Lots of code removal/simplification remains to be done and so the performance boost might increase further.

Types of Changes

Non-breaking performance improvement.

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[lhoguin]

The old index segment format is unfortunate. While the journal has 3 types of entries (message, deliver, ack), the segment file only has two (message, deliver+ack), where a deliver is indicated by having the second entry type written once, and an ack is indicated by having the second entry type written twice. The code then changes from undelivered to delivered to acked as it encounters these entries in the file when reading.

This makes any changes to deliver handling difficult. Even if the queue was tracking delivers, the index still needs to track them because of that unfortunate design, so I do not think there's much to be gained. At best we can drop the deliver writes in the journal. But the risk of doing that kind of change is important as the previous index change mess shows, and the gains while significant are not earth shattering.

I am dropping this PR as a result.

This change remains valid for the new index however.