Improve various Geo FDW queries that need aggregate pushdown to be fast

What does this MR do?

Geo needs to perform lookups against data in two separate databases. This MR improves a number of queries that are currently slow when FDW is enabled in pg 9.6

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Aggregate pushdown is only available in postgresql 10.0 or greater. This greatly slows down a number of queries. Without fast FDW-using queries, Geo is not GitLab.com-scalable.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4491 (closed)