[CASSANDRA-20226][trunk] Reduce contention in NativeAllocator.allocate#4415

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Commits on Oct 4, 2025

Reduce contention by replacing of CAS loop in MemtablePool.SubPool#tryAllocate with allocatedUpdater.addAndGet(this, size)
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Reduce contention by switching MemtableAllocator.SubAllocator#owns from updates via AtomicLongFieldUpdater to LongAdder usage.
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Commits on Oct 10, 2025

Estimate memory required to allocate for applying a partition update into a memtable and allocate this memory in one shot, then use it as a request-local SLAB
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