logstash PQ: avoid deserialization cost (POC)

What does this PR do?

Avoids deserialization cost of events passing through the PQ when memory pressure is absent, by maintaining a list of soft-references to the events added to each PQ page. When constructing batches during reads, we use a new internal intermediate type BoxedQueueable, each of which holds either a strong-reference to a Queueable or the byte[] and deserializer necessary to construct it. Deserialization, if any, still occurs outside of the Queue locking mechanism.

Why is it important/What is the impact to the user?

Reduces the overhead of reading from a PQ when a pipeline is staying caught up with the workload.

Status

This is a proof-of-concept; actual implementation may change significantly. As a proof-of-concept spike, I did not take the time to arrange it into bite-size changes.

Jul 14 '25 21:07 yaauie

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Jul 14 '25 21:07 github-actions[bot]

This pull request does not have a backport label. Could you fix it @yaauie? 🙏 To fixup this pull request, you need to add the backport labels for the needed branches, such as:

backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
If no backport is necessary, please add the backport-skip label