Implement PhutilRequest parser #2

Summary:
D6278 kind of got closed and commited, this is the actual direction.

Ref T3432

Depends on D6277

Test Plan: Keep using the site

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, mbishopim3

Maniphest Tasks: T3432

Differential Revision: https://secure.phabricator.com/D6283