PhabricatorCaches use namespace if set
Summary: PhabricatorCaches now uses namespaced caches if cofig option for cache is set.
Test Plan: noidea
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T2786
Differential Revision: https://secure.phabricator.com/D5425