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). Thefore, mixins would not be set on
the new nodes because they appeared as "already existing".