Fix file_store for artifacts and lfs when saving

What does this MR do?

This makes file_store to be updated only when file is changed, this prevents race conditions from happening during file migrations. This also makes LFS to not save LfsObject on every link to project which results in creating this race condition in the first place.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/infrastructure/issues/4115