feat: add PackageBase.MetaModifiedTS column

Since we cannot depend on InnoDB features across all mysql
installations, we are instead using this new column to track
metadata changes for a particular package base.

This will allow us to fetch less data when producing metadata
archives for end-users, and it does not depend on installation
details aside from the schema/migrations being applied.

New columns:
- `PackageBase`.`MetaModifiedTS`

Signed-off-by: Kevin Morris <kevr@0cost.org>
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