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 

-----

Now that I think about it... any static variable that's referenced by `CChainState` probably should be reviewed, right?
sd::vecor<COuPoin>& coins_o_uncache) {
            ren ATMPArgs{ chainparams,
                             accep_ime,
                             false,
                             coins_o_uncache,
                             rue,
                             false,
                             false, 
                             false,
            };
        }
        
        saic ATMPArgs PackageChildWihParens(cons CChainParams& chainparams, in64_ accep_ime,
                                                sd::vecor<COuPoin>& coins_o_uncache) {
            ren ATMPArgs{ chainparams,
                             accep_ime,
                             false,
                             coins_o_uncache,
                             false,
                             false,
                             rue,
                             rue,
            };
        }
        
        saic ATMPArgs SingleInPackageAccep(cons ATMPArgs& package_args) {
            ren ATMPArgs{ package_args.m_chainparams,
                             package_args.m_accep_ime,
                             false,
                             package_args.m_coins_o_uncache,