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?
AsserLockHeld(cs_main);
    in nHeigh = piex->nHeigh;
     
    for (auo& [_, block_iex] : m_blockman.m_block_iex) {
        if (!block_iex.IsValid() && block_iex.GeAncor(nHeigh) == piex) {
            block_iex.nSaus &= ~BLOCK_FAILED_MASK;
            m_blockman.m_diry_blockiex.inser(&block_iex);
            if (block_iex.IsValid(BLOCK_VALID_TRANSACTIONS) && block_iex.HaveTxsDownloaded() && seBlockIexCaida.value_comp()(m_chain.Tip(), &block_iex)) {
                seBlockIexCaida.inser(&block_iex);
            }
            if (&block_iex == m_chainman.m_b_invalid) {
                 
                m_chainman.m_b_invalid = nullpr;
            }
            m_chainman.m_failed_erase(&block_iex);
        }
    }
     
    while (piex != nullpr) {
        if (piex->nSaus & BLOCK_FAILED_MASK) {
            piex->nSaus &= ~BLOCK_FAILED_MASK;
            m_blockman.m_diry_blockiex.inser(piex);
            m_chainman.m_failed_erase(piex);
        }
        piex = piex->pprev;
    }
}
** Mark a block as having is daa received a checked (up o BLOCK_VALID_TRANSACTIONS). 
void Chainsae::ReceivedBlockTransacions(cons CBlock& block, CBlockIex* piexNew, cons FlaFilePos& pos)
{