`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.
assert(setIterConflicting.size() == 1);
        CTxMemPool::txiter conflict = *setIterConflicting.begin();
        m_limit_descendants += 1;
        m_limit_descendant_size += conflict->GetSizeWithDescendants();
    }
    std::string errString;
    if (!m_pool.CalculateMemPoolAncestors(*entry, setAncestors, m_limit_ancestors, m_limit_ancestor_size, m_limit_descendants, m_limit_descendant_size, errString)) {
        setAncestors.clear();
        
        std::string dummy_err_string;
        
        
        
        
        
        
        
        
        
        
        
        if (nSize >  EXTRA_DESCENDANT_TX_SIZE_LIMIT ||
                !m_pool.CalculateMemPoolAncestors(*entry, setAncestors, 2, m_limit_ancestor_size, m_limit_descendants + 1, m_limit_descendant_size + EXTRA_DESCENDANT_TX_SIZE_LIMIT, dummy_err_string)) {
            return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "too-long-mempool-chain", errString);