Geo: Race condition when updating ProjectRegistry for a wiki vs a repo

This race is probably unlikely, but may be more likely if we implement https://gitlab.com/gitlab-org/gitlab-ee/issues/9446.

• Is it more likely?
• Would it be reasonable to use a common lease key on the project, rather than use a separate key for repo and wiki sync, at least until we do https://gitlab.com/gitlab-org/gitlab-ee/issues/5499?
• What are the consequences of this race?