Loader database: the DNS trie plus a hash table of other names in use.
Make a new, empty database.
Call when guaranteed there will be no more updates.
Insert RRs in the database: args are rdata, ttl, owner, db.
Raised if we already had an RRSet for this name and type, but with a
different TTL. Also possible: Trie.BadName.
N.B. If TTLMismatch is raised, the RR was successfully added, and the
RRSet now has the new ttl.