alice writes M1 on node-1
bob writes M2 on node-2
  M1=31cd78f1-871c-4b4f-afb8-0c435972262d M2=ce46331e-e966-4867-b0ef-ca76183c2d63
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)