R1:cc3e52b8d9c9

Unfinal "DifferentialReplyHandler"

Summary: Facebook extends this, just unfinal it for now and we can figure out the right long-term fix at our leisure.

Test Plan: Inspection.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1917