alice writes M1 on node-1
bob writes M2 on node-2
  M1=6cb1d8d0-3d0a-48f4-9c0c-d520628c4bfe M2=77009059-9f1c-44f3-b45b-1485f746bde9
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)