Resolve "crash after failed ixfr-from-differences on a catalog zone"

Merged Arаm Sаrgsyаn requested to merge 3777-fix-crash-after-failed-ixfr-from-differences into main

Unregister db update notify callback before detaching from the previous db inside the catz update notify callback. Also, call the db notify callbacks only after zone_postload() returns successfully.

Closes #3777 (closed)

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