unrevert "(wangle) express current Core functionality with a state machine""

unrevert "(wangle) express current Core functionality with a state machine""

Summary: Reverts D1633874. Companion to D1636490 which fixes the bug.

Test Plan:
git reverting code that was git reverted and hasn't changed in the interim
Won't be checked in without the companion bugfix diff (D1636490)

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, net-systems@, fugalh, exa, njormrod, folly-diffs@

FB internal diff: D1636487

Tasks: 5438209

Blame Revision: D1633874