Use ArcSwap for aggregate fn registry

[robert3005]

ArcSwap is faster than a lock for read. These session are mutable but mutations
are rare and retrievals are common

[codspeed-hq]

Merging this PR will degrade performance by 10.45%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 1 regressed benchmark
✅ 1265 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	null_count_run_end[(10000, 4, 0.01)]	113.3 µs	126.5 µs	-10.45%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing rk/aggregatearcswap (a472bd2) with develop (126c431)}

[joseph-isaacs]

Shall we have a pattern like this for ArcSwap?

[joseph-isaacs]

Can we break this out into a pattern with a struct or type wrapper?

[robert3005]

I will make a follow up.

[robert3005]

Looks like this broke the backcompat check https://github.com/vortex-data/vortex/actions/runs/26544463401/job/78201950621?pr=8072. Need to investigate this tomorrow as it's passing locally