CacheStorage

Cached renderings have to be stored somewhere - why not a CacheStorage? This could be a special non-versioning, non-undo storage which examines each object it is given, and doesn't actually store it unless it is marked as especially expensive to render. Otherwise, the ordinary object cache would hold it until it became stale and the space was needed by other object. This would integrate the two sorts of caching, and eliminate the need to create and manage two cache pools.