refactor to more cpu efficient topological sort

[ptravers]

STILL VERIFYING CORRECTNESS! NOT READY FOR REVIEW!

Motivation

a request was made by @petrosagg to refactor to a more efficient algorithm in our connection topological sort.

cc: @frankmcsherry @ParkMyCar

Checklist

• [x] This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• [x] This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• [x] If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• [x] If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• [x] If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.