Improve mirror queries

I found two issues in gitlab-com/gl-infra/scalability#233 (comment 318452872):

1. We're counting archived projects.
2. We're counting public projects in private namespaces, and these do not get Gold features. I'm not even sure if they are valid: https://gitlab.slack.com/archives/CSCR2AU6S/p1586180463104200

I re-ran the queries and got:

Name	Result
mirrors_ready_to_sync	181
mirrors_not_updated_recently	0
mirrors_updated_very_recently	952
mirrors_behind_schedule	4
mirrors_scheduled_or_started	12
mirrors_scheduled	21
mirrors_started	52

I think the inconsistencies are due to mirrors that were processed after I ran one query 🙂

For gitlab-com/gl-infra/scalability#233 (closed) (again).