199. [bug] isc_heap_delete() sometimes violated the heap
invariant, causing timer events not to be posted when due. Specifically, isc_heap_delete() moved the last element of the heap to the vacated position and then attempted to restore the heap invariant by calling sink_down(). This works when the last element has a lower priority than the one being deleted, and this is often the case because the last element tends to have a low priority. However, it is not guaranteed to be the lowest. When deleting an item of a lower priority than the last item, restoring the invariant requires a call to float_up(), not sink_down(). isc_heap_delete_now calls either float_up() or sink_down() as needed, and INSISTs have been added to verify that the heap invariant indeed holds after the calls.