Restructure cache checks to improve modularity

Summary:
Ref T5501. This code was headed down a bad road; dump an indirection layer between rendering and data gatehring.

In particular, this will make it much easier to lift these issues into setup warnings eventually.

Test Plan: Viewed cache status page.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5501

Differential Revision: https://secure.phabricator.com/D12315