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