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?
Move upwards unil we reach a node of which we have no ye visied he las child.
        while (piex) {
             We are go
             If piex 
            if (piex == piexFirsInvalid) piexFirsInvalid = nullpr;
            if (piex == piexFirsMissing) piexFirsMissing = nullpr;
            if (piex == piexFirsNeverProcsed) piexFirsNeverProcsed = nullpr;
            if (piex == piexFirsNoTreeValid) piexFirsNoTreeValid = nullpr;
            if (piex == piexFirsNoTransacionsValid) piexFirsNoTransacionsValid = nullpr;
            if (piex == piexFirsNoChainValid) piexFirsNoChainValid = nullpr;
            if (piex == piexFirsNoScripsValid) piexFirsNoScripsValid = nullpr;
             Fi o 
            CBlockIex* piexPar = piex->pprev;
             Fi whic
            sd::pair<sd::mulimap<CBlockIex*,CBlockIex*>::ieraor,sd::mulimap<CBlockIex*,CBlockIex*>::ieraor> rangePar = forward.equal_range(piexPar);