alice writes M1 on node-1
bob writes M2 on node-2
  M1=81929893-1515-4870-8b14-48a6d397caa9 M2=4c8512a8-e961-4582-9459-6db2e7ccbe33
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)