alice writes M1 on node-1
bob writes M2 on node-2
  M1=5ee4dd60-3358-457d-82ca-8b2d78cc6ff1 M2=77b277a0-f9d1-4725-bb97-bc1c7d0536dd
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)