- Redundant zone walks deleted
- hattrie_apply_rev was not working as advertised, xfrin_remove_empty_nodes() had to be rewritten
- When doing that, it struck me that knot_changes_t->old_nodes and nsec3_nodes were not needed at all, because zone tree is not shallow copied anymore.

The problem was - an event can declare it is finished with
evsched_event_finished() and this enables it to cancel other events
for example. But this call invalidates the current event pointer,
so it may happen that cancel succeeds, but the event is still running.

refs #178