Addd a way to modify the Element in a DnNode

The DnNode structure is used to manage referrals, partitions and Administrative Areas cache.
In this last usage, we can only add new roles in the cache, we can't remove them, if there is some children under the node from which the role has to be removed. This is problematic as we may have to delete a role.
We need to expose a way to change the element from a node, whatever is its position in teh DnNode tree.

The SubentryServiceIT tests are failing because of that issue.