alice writes M1 on node-1
bob writes M2 on node-2
  M1=08ca13e6-a669-4083-aef6-be8efe6f8099 M2=7196b79e-3a62-4154-91a1-7f77e22d3c76
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)