validation: Review mutable static local variables in `CChainState` methods #22189

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 CChainStates 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?