s4-dsdb: operational handle modifyTimeStamp on the CN=aggregate DN

s4-dsdb: operational handle modifyTimeStamp on the CN=aggregate DN

modifyTimeStamp is a generated attribute, for most object it's generated
directly from the whenChanged attribute. But for the CN=aggregate object
in the schema we have to handle it in a different way, that's because
for this object whenChanged!=modifyTimeStamp (as checked against Windows
2003R2 DCs) instead the modifyTimeStamp reflect the timestamp of the
most recently modified and loaded schema object (that is to the one with
the highest USN before the schema was reload due to timeout or by the
reloadSchemaNow command).
Some third party are using this information to know if they have to
update their schema cache and also to check that schema updates have
been correctly reloaded by the DC, a good example of this behavior is
exchange 2010.