WIP: Remove repo from object pool upon project visibility changes

Removes repository from object pool when project visibility changes from public to private or internal

• adds a worker class that tries to put the repo into read-only, and then reduplicates and unlinks
• adds a after_save hook in the project model to kick off the worker
• adds relevant tests

What are the relevant issue numbers?

gitaly#1353 (closed)

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.
• Security reports checked/validated by reviewer