Store a limited number of full appstats records

The partial records have always been like this. The key is generated such
that only 1000 records can be stored at once, based on the start time in
ms of the request. The full keys were being stored at ns resolution, thus
storing many more. While beneficial, I'm worried that these full records
could be filling up memcache and evicting useful items. This change uses
the same key format, and thus same 1000 limit, as the partial keys.