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?
#ifndef BITCOIN_VALIDATION_H
#define BITCOIN_VALIDATION_H
#if defined(HAVE_CONFIG_H)
#include <config/bitcoin-config.h>
#endif
#include <arith_uint256.h>
#include <attributes.h>
#include <chain.h>
#include <chainparams.h>
#include <kernel/chainstatemanager_opts.h>
#include <consensus/amount.h>
#include <deploymentstatus.h>
#include <fs.h>
#include <node/blockstorage.h>
#include <policy/feerate.h>
#include <policy/packages.h>
#include <policy/policy.h>
#include <script/script_error.h>
#include <sync.h>
#include <txdb.h>
#include <txmempool.h> 
#include <uint256.h>
#include <util/check.h>
#include <util/hasher.h>
#include <util/translation.h>
#include <versionbits.h>
#include <atomic>
#include <map>
#include <memory>