Create pool repositories on Geo secondaries

Make Dedupe work with Geo

This MR adds a function into the RepositorySyncService that checks for a pool repository, and creates the underlying object pool if it doesn't exist.

Terms: pool repository - the rails model in the database object pool - the git repository used as the object pool (on disk)

We are satisfied if the state of the pool repository + object pools are eventually consistent.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9091

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer