Cache Thing contents #2
Description
The list of object IDs contained by another database object should be cached in the Thing.
If a Thing's parent is changed, it should update the contents cache of both the new and former parent Things, but only for parents that are loaded and have their contents already cached (otherwise there is no point).
Care must be taken when loading an object whose child has recently been moved elsewhere, as the database will be lagged behind the cache if the child has not yet been saved. The World class will need to maintain a sparse but authoritative index of parent-child relationships (in the form of a dict) and layer this data on top of the data returned from the database.