The live register of Prolog Web nodes. It probes each node and
publishes the result. Discovery is a query against the stateless API.
The filter is this goal, run on the hub:
Status is derived at read time from the hub's probes: a fresh success is
up; one missed probe after a good one is deemed
unreachable; a node that is alive but draining is
maintenance; and anything that remains unreachable long enough is
down.