Geo: Add partial indexes for missing_on_primary = 't' for file_registry and job_artifact_registry and later lfs_object_registry

Follow up on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5050#note_65948948

Do we need to index this column since we query it repeatedly? Probably not since it's a boolean field, but maybe a partial index for true might help.

First question: Should we do it? In that MR, I did some manual testing in GPRD and found similar queries to take less than 0.5 seconds.