Draft: chg: dev: Rewrite the database API to not expose the node-based design

The current zone and cache database design is node-centric, where each node corresponds to a DNS name. Rewrite the dns_db API in a way that tries to hide this design as much as possible and uses the name-type to manipulate the rdatasets (findrdataset, addrdataset, subtractrdataset and removerdataset), and just uses the name to index everything else (like the iterator acquire with allrdatasets).