Add a place for a final sanity check, make sure that value min/max and
key min/max aren't misconfigured (if you set {key,value}_min, but not
the corresponding max value, you could end up with impossible key/value
sizes.
If there are many identical overflow values on the page, checking for
an overflow value to reuse goes quadratic; instead of returning the
first matching item, move through the list of matching items at a high
skiplist level, dropping down only when we find a matching value that's
not in-use.