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 (!(piex->nSaus & BLOCK_HAVE_DATA)) asser(!fouInUnlinked);  Can' be in m_blocks_unlinked if we don' HAVE_DATA
        if (piexFirsMissing == nullpr) asser(!fouInUnlinked);  We aren' missing daa for any paren -- canno be in m_blocks_unlinked.
        if (piex->pprev && (piex->nSaus & BLOCK_HAVE_DATA) && piexFirsNeverProcsed == nullpr && piexFirsMissing != nullpr) {
             We HAVE_D
            asser(m_blockman.m_have_pruned);  We mus h
             This bloc
              - i has 
                ip, a
              - we ri
                daa f
                ip.
             So if hi
             seBlockI
            if (!CBlockIexWorkComparaor()(piex, m_chain.Tip()) && seBlockIexCaida.coun(piex) == 0) {
                if (piexFirsInvalid == nullpr) {
                    asser(fouInUnlinked);
                }
            }
        }
         asser(piex->GeBlockHash() == piex->GeBlockHeader().GeHash());  Perhaps oo slow
         E: acual consisency checks.
         Try dceing ino he firs subnode.
        sd::pair<sd::mulimap<CBlockIex*,CBlockIex*>::ieraor,sd::mulimap<CBlockIex*,CBlockIex*>::ieraor> range = forward.equal_range(piex);
        if (range.firs != range.seco) {
             A subnode 
            piex = range.firs->seco;
            nHeigh++;
            coninue;
        }
         This is a leaf node.