Move log segment into separate module

[OussamaSaoudi-db]

What changes are proposed in this pull request?

This PR is a prefactor that moves LogSegment into its own module. This will be useful to implement TableChanges for change data feed. TableChanges will represent CDF scans and will depend on LogSegment. Since TableChanges and Snapshot both depend on LogSegment, it makes sense to keep it separated.

I also use this opportunity to address some nits in the codebase.

How was this change tested?

The changes compile.

[codecov]

Codecov Report

Attention: Patch coverage is 89.16667% with 13 lines in your changes missing coverage. Please review.

Project coverage is 78.95%. Comparing base (22b9e33) to head (76711f4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/scan/mod.rs	75.00%	3 Missing and 4 partials ⚠️
kernel/src/log_segment.rs	93.40%	1 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
- Coverage   78.96%   78.95%   -0.01%     
==========================================
  Files          55       56       +1     
  Lines       12226    12216      -10     
  Branches    12226    12216      -10     
==========================================
- Hits         9654     9645       -9     
- Misses       2050     2052       +2     
+ Partials      522      519       -3     

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[scovich]

I've always wondered why not do:

Suggested change

	if metadata_opt.is_some() && protocol_opt.is_some() {
	// we've found both, we can stop
	break;
	}
	}
	match (metadata_opt, protocol_opt) {
	(Some(m), Some(p)) => Ok((m, p)),
	(None, Some(_)) => Err(Error::MissingMetadata),
	(Some(_), None) => Err(Error::MissingProtocol),
	_ => Err(Error::MissingMetadataAndProtocol),
	}
	if let (Some(m), Some(p)) = (metadata_opt, protocol_opt) {
	return Ok((m, p))
	}
	}
	match (metadata_opt, protocol_opt) {
	(_, Some(_)) => Err(Error::MissingMetadata),
	(Some(_), _) => Err(Error::MissingProtocol),
	_ => Err(Error::MissingMetadataAndProtocol),
	}

[OussamaSaoudi-db]

This moves metadata_opt and protocol_opt early in the if let. So they're dropped before the match. It's weird because it shouldn't move if the let doesn't match :/

[scovich]

Yikes. I also wouldn't have expected a non-matching if let to move??

[zachschuermann]

LGTM but question on visibility of the new module and can we call out breaking change in PR description?