`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 FlatFileSeq(gArgs.GetBlocksDirPath(), "rev", UNDOFILE_CHUNK_SIZE);
}
FILE* OpenBlockFile(const FlatFilePos &pos, bool fReadOnly) {
    return BlockFileSeq().Open(pos, fReadOnly);
}
static FILE* OpenUndoFile(const FlatFilePos &pos, bool fReadOnly) {
    return UndoFileSeq().Open(pos, fReadOnly);
}
fs::path GetBlockPosFilename(const FlatFilePos &pos)
{
    return BlockFileSeq().FileName(pos);
}
CBlockIndex * BlockManager::InsertBlockIndex(const uint256& hash)
{
    AssertLockHeld(cs_main);
    if (hash.IsNull())
        return nullptr;
    
    BlockMap::iterator mi = m_block_index.find(hash);
    if (mi != m_block_index.end())
        return (*mi).second;
    
    CBlockIndex* pindexNew = new CBlockIndex();
    mi = m_block_index.insert(std::make_pair(hash, pindexNew)).first;
    pindexNew->phashBlock = &((*mi).first);
    return pindexNew;
}
bool BlockManager::LoadBlockIndex(