Don't schedule ProjectCacheWorker unless needed

What does this MR do?

This MR changes ProjectCacheWorker.perform_async so scheduling only takes place when needed. See the commits for more details.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together