`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.
{
protected:
    /**
     * Every received block is assigned a unique and increasing identifier, so we
     * know which one to give priority in case of a fork.
     */
    RecursiveMutex cs_nBlockSequenceId;
    
    int32_t nBlockSequenceId = 1;
    
    int32_t nBlockReverseSequenceId = -1;
    
    arith_uint256 nLastPreciousChainwork = 0;
    /**
     * the ChainState CriticalSection
     * A lock that must be held when modifying this ChainState - held in ActivateBestChain()
     */
    RecursiveMutex m_cs_chainstate;
    /**
     * Whether this chainstate is undergoing initial block download.
     *