Use MR metrics cached data whenever available (wraps up metrics migration)

What does this MR do?

• Wraps up the migration deployed on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19097
• Create methods that use the cached data on MergeRequest, falling back to events table when needed

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides