It represents which probe the _thing_ was seen from, but in many cases
(container images, deployments, replicasets, services), it may have come
from several probes. We have previously conflated it to determine which
host a thing _lives on_, which it may not even have (deployments,
replica sets, services), or it may have multiple (container images).
The idea is to separate those two usages. We should convert HostNodeID
to a set of HostNodeIDs, and use that to determine which probes have
reported the thing. For determining which host a thing lives on we
should use the Parents field which we already have, but might need
extending to handle Endpoints/etc...
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