alice writes M1 on node-1
bob writes M2 on node-2
  M1=fe6297f9-dd3b-4869-9c11-02c51ec46131 M2=e6f708e1-91bd-49be-bd12-7c9a1731bfc5
settle 5s for pre-link replication
alice links M1 -> M2 with relation=related_to
  link POST returned HTTP 201
settle 8s for link fanout
charlie queries links of M1 on node-3
  charlie sees M1->M2 link: 1 (expected >=1)