Geo: Verification of LFS objects

Data integrity is essential to Geo. If the data is worth replicating, then it's worth verifying.

• Wait for attachment reverification to be completed https://gitlab.com/gitlab-org/gitlab-ee/issues/5591 Edit: The former issue was modified over time to only cover front-end. Now this depends on https://gitlab.com/gitlab-org/gitlab-ee/issues/7257
• Automatically verify LFS objects as well

Refactor if needed, don't reimplement things.

cc @harishsr @rnienaber