New sorted-dup subpage support
Instead of converting directly to a subDB when the first duplicate item is seen for a key, convert to a subpage instead. Allow the subpage to grow up to the overflow limit, then convert to a subDB. This saves a significant amount of space in a typical slapd index database. Currently we don't convert back to the smaller form if items are later deleted. Probably could do that with some hysteresis, e.g., convert back from subDB to subpage when the size drops below (overflow limit/2). Maybe later.