alice writes M1 on node-1
bob writes M2 on node-2
  M1=f4574b7f-420b-4305-a164-452386643717 M2=80956b3a-25cb-47b4-abcc-fe2afe05ebcb
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)