Skip already rehashed positions in the old hashmap table

When iterating through the old internal hashmap table, skip all the
nodes that have been already migrated to the new table.  We know that
all positions with index less than .hiter are NULL.

lib/isc/hashmap.c

@@ -646,7 +646,7 @@ isc__hashmap_iter_next(isc_hashmap_iter_t *iter) {
 
 	if (try_nexttable(hashmap, iter->hindex)) {
 		iter->hindex = hashmap_nexttable(iter->hindex);
-		iter->i = 0;
+		iter->i = hashmap->hiter;
 		iter->size = hashmap->tables[iter->hindex].size;
 		return (isc__hashmap_iter_next(iter));
 	}