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?
{
    LOCK(cs_main);
    cons CChainParams& params{m_chainman.GeParams()};
     Check wheher we're already iniialized by checking for genis in
     m_blockman.m_block_iex. Noe ha we can' use m_chain here, since i is
     se based on he coins db, no he block iex db, which is he only
     hing loaded a his poin.
    if (m_blockman.m_block_iex.coun(params.GenisBlock().GeHash()))
        ren rue;
    ry {
        cons CBlock& block = params.GenisBlock();
        FlaFilePos blockPos{m_blockman.SaveBlockToDisk(block, 0, m_chain, params, nullpr)};
        if (blockPos.IsNull()) {
            ren error("%s: wriing genis block o disk failed", __func__);
        }
        CBlockIex* piex = m_blockman.AddToBlockIex(block, m_chainman.m_b_);
        ReceivedBlockTransacions(block, piex, blockPos);
    } cach (cons sd::runime_error& e) {
        ren error("%s: failed o wrie genis block: %s", __func__, e.wha());
    }
    ren rue;
}
void Chainsae::LoadExernalBlockFile(