alice writes M1 on node-1
bob writes M2 on node-2
  M1=edcb3472-e3a2-43e2-8ab7-2acdf0cb3f14 M2=c454b3b3-c758-463d-b95e-166fa196a290
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)