Fix the dbiterator to assume only one qp-trie

Fix the dbiterator to assume only one qp-trie

The dbiterator can take three modes: full, nsec3only and nonsec3.
Previously, in full mode the dbiterator requires special logic to jump
from one qp-trie to the other. Now everything is in one trie, other
special logic is needed.

The qp-trie is now sorted in such a way that all the normal nodes come
first, followed by NSEC nodes, and finally the NSEC3 nodes. NSEC nodes
are empty nodes and need to be skipped when iterating.

We add an additional auxiliary node to the trie, an NSEC origin, so
we can easily find the point in the trie where we need to continue
iterating.