dbchecker: verify and fix broken dn values

dbchecker: verify and fix broken dn values

With older Samba versions (4.0.x) the following could happen:

- On account was created on DC1
- It was replicated to DC2
- The connection between the dcs is offline
- The account gets modified on DC2
- The account gets deleted on DC1
- The connection becomes online again
- DC1 replicates the modification from DC2,
  this resets the dn to the original value.
  'name' and 'cn' are correct (with '\nDEL${GUID}'),
  but 'dn' is wrong.
- DC2 replicates the deletion from DC1.
  this doesn't include a changed dn as DC1
  had a bug.
  'name' is correct (with '\nDEL${GUID}'),
  but 'cn' and 'dn' are wrong.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10536
Change-Id: Ia70a6c12e0ff0d4c2c8100cb1d8f3c6422b65591
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>