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