Use the changeset parse cache to cache suggestion changesets

Summary:
Ref T13513. Syntax highlighting is potentially expensive, and the changeset rendering pipeline can cache it. However, the cache is currently keyed ONLY by Differential changeset ID.

Destroy the existing cache and rebuild it with a more standard cache key so it can be used in a more ad-hoc way by inline suggestion snippets.

Test Plan: Used Darkconsole, saw cache hits and no more inline syntax highlighting for changesets with many inlines.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21280