`RewindBlockIndex` was made to allow nodes upgrading to new consensus rules post-segwit fork to 
download new consensus data (i.e witnesses) without having to go through a whole IBD. It has been pointed than now really few pre-segwit nodes care to upgrade and that this logic may be irrelevant.

See https://github.com/bitcoin/bitcoin/pull/17737#discussion_r358514335 in review  of #17737 for more context pointers.
return false;
        }
        CAutoFile file(filestr, SER_DISK, CLIENT_VERSION);
        uint64_t version = MEMPOOL_DUMP_VERSION;
        file << version;
        file << (uint64_t)vinfo.size();
        for (const auto& i : vinfo) {
            file << *(i.tx);
            file << int64_t{count_seconds(i.m_time)};
            file << int64_t{i.nFeeDelta};
            mapDeltas.erase(i.tx->GetHash());
        }
        file << mapDeltas;
        LogPrintf("Writing %d unbroadcast transactions to disk.\n", unbroadcast_txids.size());
        file << unbroadcast_txids;
        if (!skip_file_commit && !FileCommit(file.Get()))
            throw std::runtime_error("FileCommit failed");
        file.fclose();
        if (!RenameOver(GetDataDir() / "mempool.dat.new", GetDataDir() / "mempool.dat")) {
            throw std::runtime_error("Rename failed");
        }
        int64_t last = GetTimeMicros();
        LogPrintf("Dumped mempool: %gs to copy, %gs to dump\n", (mid-start)*MICRO, (last-mid)*MICRO);
    } catch (const std::exception& e) {
        LogPrintf("Failed to dump mempool: %s. Continuing anyway.\n", e.what());
        return false;
    }
    return true;
}
double GuessVerificationProgress(const ChainTxData& data, const CBlockIndex *pindex) {