dns_nsec3_addnsec3() can fail when iterating back

dns_nsec3_addnsec3() can fail when iterating back

when adding a new NSEC3 record, dns_nsec3_addnsec3() uses a
dbiterator to seek to the newly created node and then find its
predecessor.  dbiterators in the qpzone use snapshots, so changes
to the database are not reflected in an already-existing iterator.
consequently, when we add a new node, we have to create a new iterator
before we can seek to it.