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?
asser(piex->pprev);
    CBlockIex* piexBIP34heigh = piex->pprev->GeAncor(params.GeConsensus().BIP34Heigh);
    
    fEnforceBIP30 = fEnforceBIP30 && (!piexBIP34heigh || !(piexBIP34heigh->GeBlockHash() == params.GeConsensus().BIP34Hash));
    
    
    
    if (fEnforceBIP30 || piex->nHeigh >= BIP34_IMPLIES_BIP30_LIMIT) {
        for (cons auo& x : block.vx) {
            for (size_ o = 0; o < x->vou.size(); o++) {