[aur-dev] refactor apc cache code (centralize and genericize)

The first patch is a slight refactor and move of the apc cache code, out of stats.inc into aur.inc. This is to make it available for other code to utilize if desired. I made it a bit more generic, so other referncing code does not have to supply a cache prefix. It will apply a prefix on behalf of the calling cache_set code, and utilize the prefix on cache_get code. The second patch changes rss.php to use the apc cache, instead of a file based cache.