Rework dbiterator implementation

Rework dbiterator implementation

If the iterator is paused, the tree is unlocked and may change.

In an RBT tree it's always possible to resume iteration as long
as a valid node pointer was still held, but now that the underlying
database structure is a QP trie, the iterator needs to be initialized
based on the existing structure of the trie or it will return
inconsistent results. We now call dns_qp_lookup() to reinitialize
the QP iterator whenever dbiterator_next() or dbiterator_prev() is
called on a paused iterator.