Fix broken syncing tests

Summary: In an epic feat of backpedaling, I realized that the "refactors" I made to the syncing algorithm made it susceptible to a race condition. I reverted the fixes and the tests now pass.

Test Plan: Ran the tests

Reviewers: emfree

Reviewed By: emfree

Subscribers: spang

Differential Revision: https://review.inboxapp.com/D770