alice writes M1 on node-1
bob writes M2 on node-2
  M1=121c6ed4-e588-477e-a796-39eeaa135197 M2=a1b2f490-96a9-491b-8389-7332a886501a
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)