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

[elij]

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.