Give HarbormasterBuildLogChunk a real table
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.
Reviewed By: chad
Maniphest Tasks: T10457
Differential Revision: https://secure.phabricator.com/D15376