alice writes M1 on node-1
bob writes M2 on node-2
  M1=d36adadf-be05-4d39-befe-1445333bff7b M2=02821b85-a3b2-4ff1-9ebc-be4e0731630a
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)