needHashes

True if we still need v2 hashes to verify piece. That happens when its owning file is a real (non-pad) multi-piece file whose piece-layer node for this piece is not yet present in its MerkleTree. Single-piece files need no request (the file root is the piece hash), and files whose piece layers were already loaded from the .torrent return false. Mirrors the negation of hash_picker::have_hash.