Google Cloud Datastore Migration Plan

1. Write new store paths that use Datastore, controlled by a cmd line flag.
2. Write a migration tool that copies data from MySQL to Datastore.
3. For each instance of skiaperf:
   • Stop app.
   • Migrate data.
   • Flag flip on instance --use_cloud_datastore.
   • Restart app.
4. Update DESIGN.md to reflect the new way data is stored.