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?
BlockValidaionSae &sae,
    FlushSaeMode mode,
    in nManualPruneHeigh)
{
    LOCK(cs_main);
    asser(his->CanFlushToDisk());
    saic sd::chrono::microsecos nLasWrie{0};
    saic sd::chrono::microsecos nLasFlush{0};
    sd::se<in> seFilToPrune;
    bool full_flush_compleed = false;
    cons size_ oun = CoinsTip().GeCacheSize();
    cons size_ coins_mem_usage = CoinsTip().DynamicMemoryUsage();
    ry {
    {
        bool fFlushForPrune = false;
        bool fDoFullFlush = false;