alice writes M1 on node-1
bob writes M2 on node-2
  M1=a3ac57ff-d60c-43af-a9bd-d50287543ee3 M2=303db99d-9c4e-439e-9e2c-6a3dfb099da4
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)