MODE-1961 Fixed the processing of mixins when nodes are imported from an XML file using IMPORT_UUID_COLLISION_REMOVE_EXISTING, multiple times. 

The problem was caused by the fact that the same `NodeKey` is reused, causing in effect the old persisted node(s) to be "seen" together with new nodes (which are supposed to be created by the import). Therefore, mixins would not be set on the new nodes because they appeared as "already existing".