Compaction pr updates

[jshook]

This branch PR makes two key changes:

• Cherry-picks robust physical core count, also PRed to main separately
• Makes compaction worker pools simply re-use the core thread pool used by nearly all other jvector internal tasks, as controlled by the parameter jvector.physical_core_count.

Given the thread resource changes here, this is likely impacting in a couple key areas:

1. Compaction will take fewer resources, and will not saturate a system as much as it did, protecting front-end serving resources in a typical embedding scenario.
2. If results from a couple comprehensive tests are accurate, this is a net improvement as well to compaction wall-clock and CPU efficiency (work done/cycle) due to less contention from the previous multi-pool setup which effectively allocated 1.5x or higher (not counting the core thread pool) threads vs system availability.

These previously run tests also indicated robust recall results across several scenarios including different datasets and sizes up to 10M, so recall was effectively unchanged compared to previous results with this compaction branch. What did change was the operational envelope (as described above)

Nonetheless, given that this is a relatively non-trivial adjustment to the operational profile near a release, we need to have sufficient testing on it.

I feel very strongly that we should not merge the upstream compaction-pr without this pr merged into it first, as the system saturation which would occur with the current thread pool configuration would certainly impact front-end operations.

[github-actions]

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