Allow Memcache Statistics on JSON Feeds

Testing Steps

As a site administrator who can access the memcache settings, verify that you have the new setting for "Show memcache statistics at the bottom of json output"
Verify you can enable this setting and it can be saved

Open up a JSON feed with access to view memcache admin stats, verify you are able to see the stats at the bottom of the feed

Verify these stats go away when the admin option is unchecked
Verify there are no adverse effects for users with no permissions