r18781: Move the usnCreated and usnChanged handling around again.

r18781: Move the usnCreated and usnChanged handling around again.

This moves these attributes from objectguid into an optional backend
(objectguid), used by ltdb.  For OpenLDAP, the entryUUID module
converts entryCSN into usnChanged.

This also changes the sequence number API, and uses 'time based'
sequence numbers, when an LDAP or similar backend is detected.

To assist this, we also store the last modified time in the TDB,
whenever we change a value.

Andrew Bartlett
(This used to be commit 72858f859483c0c532dddb2c146d6bd7b9be5072)