R1:4f42d85c1c09

Use a serialize()-based cache instead of a JSON-based cache for changesets

Summary: These are full of PhutilSafeHTML objects now, which are destroyed by JSON serialization.

Test Plan: Dropped cache, then reloaded pages.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4942