Allow files to TTL and and be garbage collected

Summary: Added ttl field to files. Gabage collect files with expired ttl

Test Plan: created file with a ttl. Let garbage collector run

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4987

resources/sql/patches/20130215.phabricatorfileaddttl.sql

@@ -0,0 +1,3 @@
+ALTER TABLE {$NAMESPACE}_file.file 
+  ADD ttl INT(10) UNSIGNED DEFAULT NULL, 
+  ADD KEY key_ttl (ttl);