`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.
nBlockSequenceId = 1;
    setBlockIndexCandidates.clear();
}
void UnloadBlockIndex(CTxMemPool* mempool, ChainstateManager& chainman)
{
    LOCK(cs_main);
    chainman.Unload();
    pindexBestInvalid = nullptr;
    pindexBestHeader = nullptr;
    if (mempool) mempool->clear();
    vinfoBlockFile.clear();
    nLastBlockFile = 0;
    setDirtyBlockIndex.clear();
    setDirtyFileInfo.clear();
    versionbitscache.Clear();
    for (int b = 0; b < VERSIONBITS_NUM_BITS; b++) {
        warningcache[b].clear();
    }
    fHavePruned = false;
}
bool ChainstateManager::LoadBlockIndex(const CChainParams& chainparams)