Improve compatibility of "Config > Cache Status" across APCu versions

Summary:
Ref T13164. See PHI790. Older versions of APCu reported cache keys as "key" from apcu_cache_info(). APC and newer APCu report it as "info".

Check both indexes for compatibility.

Test Plan:

• Locally, with newer APCu, saw no behavioral change.
• Will double check on admin, which has an older APCu with the "key" behavior.
• (I hunted this down by dumping apcu_cache_info() on admin to see what was going on.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19569