sec: usr: Reclaim memory promptly when DNSSEC validations are canceled

When a resolver is flooded with queries that require DNSSEC
validation — for example during a random-subdomain attack —
many of those validations are canceled before they complete.
Previously a canceled validation still kept its place in the
internal work queue and held the associated response in memory
until that queued work eventually ran, so memory could climb
sharply under sustained load. The canceled work is now dropped
as soon as the validation is canceled, releasing the memory it
was holding.

Merge branch '4760-cancel-dns_validator-jobs-early' into 'security-main'

See merge request isc-private/bind9!1092