ObjectStorage::Concern#migrate! incurs data loss

Summary

The ObjectStorage::Concern#migrate! method might cuase data loss in some edge cases.

It seems to be linked to the move_to_store and move_to_cache not being correctly handled by the #migrate! method.

See https://gitlab.com/gitlab-com/infrastructure/issues/3674 for context.

Possible fixes

Make sure the #migrate! method is atomic.

(If you can, link to the line of code that might be responsible for the problem)