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.