Fix wrong use of ActiveRecord in PoolRepository

What does this MR do?

We used a ActiveRecord has_one association instead of belongs_to in PoolRepository, which lead to incorrect behavior in Git object deduplication.

This MR fixes the association. I have left a follow-up item in gitaly#1625 (closed) to add a data migration to fix invalid records that were created by the old code. The impact of invalid records is that you probably do not get the space savings implied by Git object deduplication. I think this is worth fixing but not critical. The code that creates the bad records is behind a feature flag (object_pools) that is off by default in GitLab 11.10.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team