Give HarbormasterBuildLogChunk a real table

Summary:
Ref T10457. Currently, this table is an ad-hoc table, but can easily be turned into a normal table.

This will make iterating over log chunks to compress and archive them easier.

Test Plan: Viewed logs, ran bin/storage adjust with no issues.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10457

Differential Revision: https://secure.phabricator.com/D15376