perf: raise default max frame size to 64 KiB

[anmonteiro]

Summary

• raise the default Config.read_buffer_size / advertised SETTINGS_MAX_FRAME_SIZE from 16 KiB to 64 KiB
• update config docs to match the new default
• make the affected tests explicit about whether they rely on the protocol minimum or the library default

Why

On the large-upload benchmark, the server was still advertising the protocol-minimum MAX_FRAME_SIZE (16 KiB). That forces clients such as curl/nghttp2 to fragment a 1.5 GiB upload into a very large number of DATA frames, which amplifies parser, flow-control, and write-loop overhead.

64 KiB was the best point I measured locally on this workload:

• 16 KiB: much slower
• 64 KiB: best throughput
• 256 KiB and 1 MiB: slightly worse again

So this is not just “bigger is better”; 64 KiB looks like the right default tradeoff here.

Validation

• dune build --display=short @runtest-test_h2 @runtest-test_h2_client @runtest-test_h2_server spec/eio_h2spec.exe
• all tests passed

Benchmark

Using the existing 1.5 GiB curl upload benchmark against the Eio server, with the same local gluten checkout in both variants:

• baseline (perf/batch-window-updates): 3.391s, 452.94 MiB/s; 3.409s, 450.62 MiB/s
• 64 KiB default max frame size: 0.813s, 1890.22 MiB/s; 0.799s, 1921.55 MiB/s

This is roughly a 4.2x throughput improvement on this workload.

Notes

This changes the default connection preface behavior: default connections now send a non-empty SETTINGS frame advertising a larger MAX_FRAME_SIZE. The tests updated here cover the two places that were implicitly assuming the old 16 KiB default.