[9.16] Resolve "ThreadSanitizer: data race lib/dns/rbtdb.c:1365 in newversion"

Arаm Sаrgsyаn requested to merge 3907-data-race-in-rbtdb-v9_16 into v9_16

Backport of !7637 (merged).

FTR: There are commits in !7637 (merged), which make sense for main (and for v9_18 too), because they now process catalog zone updates with catzs->lock unlocked, and they use "offload" threads for the update.

Those commits were not backported, because they are not useful in 9_16.

Closes #3907 (closed)

Edited by Arаm Sаrgsyаn

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