Fix for 3525 : Attribute modifications are not replicated for modDN operations

Whenever a MODDN or MODRDN operation is replicated, the
core server does not take into account the modifications that are
provided by the replication. Therefore operational attribute
such as modifiersname and modifytimestamp are  lost.

This fix change the behaviour of the server so that
- it does not calls pre-operation plugins for synchronization operation
  (as it is done for the other operations)
- it takes into account the provided modifications when the operation
  is a synchronization operation