More changes to improve the speed of CommitStatsRecalculator

- Add index on commits_keys.key_id (yeah...)
- Rather than simply spawn a new CSR for every commit in a project when a
  commit is imported, a fast way of actually calculating which commits would
  have been affected by the imported commit
- ... followed by the realization that because of ProjectTranslationAdder,
  chances are the readiness of keys will never be affected by an import, and
  thus it's not necessary to run CSR's for any commit except the one that was
  just imported. So, commented all that code out.