`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.
public:
    CVerifyDB();
    ~CVerifyDB();
    bool VerifyDB(const CChainParams& chainparams, CChainState& active_chainstate, CCoinsView *coinsview, int nCheckLevel, int nCheckDepth) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
};
enum DisconnectResult
{
    DISCONNECT_OK,      
    DISCONNECT_UNCLEAN, 
    DISCONNECT_FAILED   
};
class ConnectTrace;
enum class FlushStateMode {
    NONE,
    IF_NEEDED,
    PERIODIC,
    ALWAYS
};
struct CBlockIndexWorkComparator
{
    bool operator()(const CBlockIndex *pa, const CBlockIndex *pb) const;
};
/**
 * Maintains a tree of blocks (stored in `m_block_index`) which is consulted
 * to determine where the most-work tip is.
 *
 * This data is used mostly in `CChainState` - information about, e.g.,
 * candidate tips is not maintained here.
 */
class BlockManager