The previous code here simply threw away whatever it knew about cache
entry ages whenever a counter overflow occurred.  Since the counter
is int width and will be bumped once per format function execution,
overflows are not really so rare as to not be worth thinking about.
Instead, let's deal with the situation by halving all the age values,
essentially rescaling the age metric.  In that way, we retain a
pretty accurate (if not quite perfect) idea of which entries are oldest.