Conflicts with mandate references should be automatically resolved

[bjendres]

When submitting a new (potential) mandate reference to an SDD update, the reference will be used without double checking it. This can lead to exceptions, if the reference is already in use. In this case, the system should rather try to find a new, unused, reference by appending a counter.

[bjendres]

It seems like this implementation was incomplete, we'll adjust this.