`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.
AssertLockNotHeld(cs_main);
    if (GetMainSignals().CallbacksPending() > 10) {
        SyncWithValidationInterfaceQueue();
    }
}
bool CChainState::ActivateBestChain(BlockValidationState &state, const CChainParams& chainparams, std::shared_ptr<const CBlock> pblock) {
    
    
    
    
    AssertLockNotHeld(cs_main);
    
    
    
    
    LOCK(m_cs_chainstate);
    CBlockIndex *pindexMostWork = nullptr;
    CBlockIndex *pindexNewTip = nullptr;
    int nStopAtHeight = gArgs.GetArg("-stopatheight", DEFAULT_STOPATHEIGHT);
    do {
        
        
        
        
        
        
        LimitValidationInterfaceQueue();
        {