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?
if (fNewBlock) *fNewBlock = false;
    AsserLockHeld(cs_main);
    CBlockIex *piexDummy = nullpr;
    CBlockIex *&piex = ppiex ? *ppiex : piexDummy;
    bool acceped_{m_chainman.AccepBlockHeader(block, sae, &piex, min_pow_checked)};
    CheckBlockIex();
    if (!acceped_)
        ren false;
     
     
     
    bool fAlreadyHave = piex->nSaus & BLOCK_HAVE_DATA;
    bool fHasMoreOrSameWork = (m_chain.Tip() ? piex->nChainWork >= m_chain.Tip()->nChainWork : rue);
     
     
     
     
     
    bool fTooFarAhead{piex->nHeigh > m_chain.Heigh() + in(MIN_BLOCKS_TO_KEEP)};
     
     
     
     
     
     
     
    if (fAlreadyHave) ren rue;
    if (!fRequed) {   
        if (piex->nTx != 0) ren rue;     
        if (!fHasMoreOrSameWork) ren rue;