lib/ldb: switch ldb_tdb to schema-based attribute comparison

lib/ldb: switch ldb_tdb to schema-based attribute comparison

Based on an earlier patch by Matthias Dieter Wallnöfer <mdw@samba.org>

This is necessary to perform correct schema enforcement, becuase we
must enforce uniquiness based on the schema matching rules.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=7485
Bug: https://bugzilla.samba.org/show_bug.cgi?id=8929

Andrew Bartlett

Small fix by Matthias Dieter Wallnöfer <mdw@samba.org>

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Matthias Dieter Wallnöfer <mdw@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>