`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.
{
private:
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    std::unique_ptr<CChainState> m_ibd_chainstate GUARDED_BY(::cs_main);
    
    
    
    
    
    
    
    
    
    std::unique_ptr<CChainState> m_snapshot_chainstate GUARDED_BY(::cs_main);