Summary:
This implements proposal #2 in our State architecture doc: https://fb.quip.com/bm2EAVwL7jQ5
Problem description: see the text in the comment of TreeStateReconciliation.h
Solution: see also comments in TreeStateReconciliation.h.
Changelog: [internal]
Reviewed By: mdvacca
Differential Revision: D19617329
fbshipit-source-id: 845fb5fe27f2591be433b6d77799707b3516fb1a