Resolve "Slower sync when backoff time is being respected"

What does this MR do?

This change allows picking new projects for sync despite retry_at set to the future when there is a capacity for that.

Some other options were considered in https://gitlab.com/gitlab-org/gitlab-ee/issues/5348

This MR only touches FDW queries as for legacy query it comes at a cost and we aim to remove them soon (12.1).

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #5348 (closed)