Maintainer page optimization

[AMDmi3]

P99 is above 1sec, should be optimized.

• Parallel fetch of projects and similar maintainers
• Measure projects and similar maintainers query time

It's already known though that similar maintainers query is rather slow since it needs to process a lot of maintainer_metapackages (to get all maintainers with intersecting projects list), then fetch maintainer info for all of these.

• May try to optimize it by passing numeric maintainer id to the query to avoid InitPlans and possibly make it easier for an optimizer
• Unwrap subqueries into CTEs (should have no effect, but improve query readability)

Real optimization would need prepared data, such as maintainer_id/project_id/num_maintainer_projects table which is smaller and does not require join with maintainers, or similar maintainers percomputed totally offline.

For now, reducing maintainer_metapackages should also help, such as by dropping unneeded maintainers (#121), and giving more memory to the database, as maintainer_metapackages is quite hot and should be well cached.