Decouple cache key prefix from getter function.

Instead of adding a cache key prefix, it would work better to use
WordPress' cache "group" concept. This provides ample prefixing and
reduces the overall size of the cache key string.
Initialize the object cache.

Load the object cache for use with the page cache. WordPress loads the
object cache too late for use in the page cache. Loading it in
advanced-cache.php allows for the page cache to take advantaged of the
object cache.