[9.18] prevent query_coveringnsec() from running twice

Evan Hunt requested to merge 3835-cname-wildcard-loop-followup-bind-9.18 into bind-9.18

when synthesizing a new CNAME, we now check whether the target matches the query already being processed. if so, we do not restart the query; this prevents a waste of resources.

(cherry picked from commit 0ae8b2e0)

Backport of MR !8214 (merged)

Edited by Evan Hunt

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