Add commitlog knobs to server config

[joshua-spacetime]

Description of Changes

Adds the commitlog knobs max_segment_size, write_buffer_size, and preallocate_segments to the server's config.toml. There are other seemingly more advanced knobs that I did not add at this time.

This patch also increases the DEFAULT_WRITE_BUFFER_SIZE from 8KiB to 128KiB to optimize high throughput workloads like the keynote-2 benchmark.

API and ABI breaking changes

None

Expected complexity level and risk

1

Testing

Manual

[kim]

The change is fine by me, but I'll note that commitlog config is exposed in the other edition, too, under a different key. It is probably not a good idea to let the config.toml diverge unless we have a good reason.

Also note that there is docs/docs/00300-resources/00200-reference/00100-cli-reference/00200-standalone-config.md, which I think should reflect these settings, even if they're deemed unstable.