Add wait for setup issue fallback disk cache

Summary: See D5657. Also cleans up the namespacing stuff a little bit.

Test Plan: Disabled APC and verified that setup checks didn't run normally, but did run after restart and on /config/issue/.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5658