When looking up a zonecut in cache, we use 'dns_rbt_findnode' to find
the closest matching node. This function however does not take into
account stale nodes. When we do find a stale node and use it, this
has implications for subsequent lookups. For example, this may break
QNAME minimization because we are using a deeper zonecut than we should
have.
Check the header for staleness and if so, and stale entries are not
accepted, look for the deepest zonecut from this node up.
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