store dataset metadata differently

[rompetroll]

currently, we store metadata about datasets in a special, not exposed dataset, called "core.Dataset".

Since this is a dataset, all changes to entities in it are tracked as dataset changes. this is not interesting data to collect, so we should not store this meta information in a dataset - but rather as separate objects in a system index/key.
This way, we can overwrite the metadata objects as much as we like without building a change history.