Change the type of entry.Free to address.Count

This change is backward compatible, because the underlying types of
address.Offset (the previous type) and address.Count are the same
and the Gob encoder does not require types to exactly correspond.

net/address/address.go

@@ -151,7 +151,7 @@ func Length(a, b Address) Count {
 	return Count(a - b)
 }
 
-func Min(a, b Offset) Offset {
+func Min(a, b Count) Count {
 	if a > b {
 		return b
 	}