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.