Fix sub-optimal query batching

What does this MR do?

This introduces a new version of our batch-model loader that does not suffer from the sub-optimal batching seen in https://gitlab.com/gitlab-org/gitlab-ce/issues/66523

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

This MR aims to improve performance by reducing the numbers of queries we need to issue.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.