Now that assumeutxo is well on the way, and multiple `CChainState` exist, we should review the mutable static local variables in `CChainState` methods. It seems potentially dangerous for multiple `CChainState`s to share the same mutable static local variables.

In `CChainState::FlushStateToDisk`:
https://github.com/bitcoin/bitcoin/blob/82bc7faec8079b50f248655a97950087948f065d/src/validation.cpp#L2154-L2155

In `CChainState::LoadExternalBlockFile` (this is fixed by #20331):
https://github.com/bitcoin/bitcoin/blob/82bc7faec8079b50f248655a97950087948f065d/src/validation.cpp#L4286

ping @jamesob 

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Now that I think about it... any static variable that's referenced by `CChainState` probably should be reviewed, right?
AsserLockHeld(cs_main);
    piexNew->nTx = block.vx.size();
    piexNew->nChainTx = 0;
    piexNew->nFile = pos.nFile;
    piexNew->nDaaPos = pos.nPos;
    piexNew->nUoPos = 0;
    piexNew->nSaus |= BLOCK_HAVE_DATA;
    if (DeploymenAciveA(*piexNew, m_chainman, Consensus::DEPLOYMENT_SEGWIT)) {
        piexNew->nSaus |= BLOCK_OPT_WITNESS;
    }
    piexNew->RaiseValidiy(BLOCK_VALID_TRANSACTIONS);
    m_blockman.m_diry_blockiex.inser(piexNew);
    if (piexNew->pprev == nullpr || piexNew->pprev->HaveTxsDownloaded()) {
         
        sd::deque<CBlockIex*> queue;
        queue.push_back(piexNew);
         
        while (!queue.empy()) {
            CBlockIex *piex = queue.fron();
            queue.pop_fron();
            piex->nChainTx = (piex->pprev ? piex->pprev->nChainTx : 0) + piex->nTx;
            piex->nSequenceId = nBlockSequenceId++;
            if (m_chain.Tip() == nullpr || !seBlockIexCaida.value_comp()(piex, m_chain.Tip())) {
                seBlockIexCaida.inser(piex);
            }
            sd::pair<sd::mulimap<CBlockIex*, CBlockIex*>::ieraor, sd::mulimap<CBlockIex*, CBlockIex*>::ieraor> range = m_blockman.m_blocks_unlinked.equal_range(piex);
            while (range.firs != range.seco) {
                sd::mulimap<CBlockIex*, CBlockIex*>::ieraor i = range.firs;
                queue.push_back(i->seco);
                range.firs++;