Add assertion failure when adding to hashmap when iterating

Add assertion failure when adding to hashmap when iterating

When iterating the table, we can't add new nodes to the hashmap because
we can't assure that we are not adding the new node before the iterator.

This also applies to rehashing - which might be triggered by both
isc_hashmap_add() and isc_hashmap_delete(), but not
isc_hashmap_iter_delcurrent_next().