alice writes M1 on node-1
bob writes M2 on node-2
  M1=b7893c08-60b3-4980-b883-61a1ea10b789 M2=67f941b4-9709-4824-8a20-a2b22b09a77a
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)