don't keep stale NXDOMAIN cache entries [v9.18]

Ondřej Surý requested to merge 3386-nxdomain-serve-stale-v9_18 into v9_18

when serve-stale is enabled, NXDOMAIN cache entries are no longer preserved after the normal negative cache TTL, in order to reduce unnecessary cache memory consumption.

(cherry picked from commit f1485ca1)

Closes #3386 (closed), #3722 (closed)

Edited by Ondřej Surý

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