commitlog: Change default max-records-in-commit to 1

[kim]

It is almost always wrong / undesirable to pack more than one transaction in a commit, so adjust the default accordingly.

This also avoids surprises when using #[serde(default)] with nested structs -- serde evaluates the default depth-first, so overriding a single field in a nested struct will not consider any #[serde(default = "custom_default")] annotations on the parent.