Bigger split sizes for aggregate scans

[myrrc]

Aggregate splits reading from zone maps don't decode columns so current split
size creates overhead for them (clickbench q6, vortex-bench q3). Set 4 big
splits per code to mitigate this overhead

[codspeed-hq]

Merging this PR will not alter performance

✅ 1841 untouched benchmarks
⏩ 55 skipped benchmarks¹

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_{Comparing myrrc/zonemap-stats-aggregate-scan-splits (62d1e8a) with zonemap-stats-layout (615d8ce)}

Footnotes

1. 55 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩