alice writes M1 on node-1
bob writes M2 on node-2
  M1=b1ed4831-09a8-4ca7-8d52-6d352912a83e M2=3a14f4fb-3830-43b2-a966-fd3cec4c31d6
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)