core: fix temp memory blowup caused by defers holding on to state #22319
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When adding the background trie prefetcher, I've used defers to make sure the prefetchers are stopped irrelevant what error path the importer exits on. Learnt once again that defer in a loop is a horrible idea, because it kept references to old state imported in the same batch. On the Shanghai attack range, this caused a temporary memory blowup, which if enough blocks were imported in a single batch could cause a significant RAM spike.
Shoutout to Michael for finding this issue.