<!-- Describe the issue -->

Hi everyone,
We use bitcoin core with hundreds of wallets. The only efficient way we have found to do so is :
1. wait for any bitcoin-cli command to terminate before issuing another one
2. never issue any bitcoin-cli command unless tip update is 1
3. always unload a wallet after loading and using it

Now, we observ that bitcoin core systematically crashes every 20-30 minutes, then automatically restarts. And the problem is, it has to re-sync from about 0.997, which easily takes 15-20 minutes. During that time, the daemon cannot be used.

If we don't load/unload wallets, the daemon is stable and keeps to 1, no issue.

**System information**
Bitcoin Core RPC client version v0.20.0.0-ga62f0ed64f8bbbdfe6467ac5ce92ef5b5222d1bd
Compiled and run on Debian 9, on a VM with 16 mo memory. Launched that way : /bitcoind -daemon -walletrbf -conf=/root/.bitcoin/bitcoin.conf -fallbackfee=0.00001 -rpcworkqueue=128 -txindex=1

**bitcoin.conf :**
server=1
dbcache=16384
par=1
blocksonly=1
maxuploadtarget=137
maxconnections=16
rpcuser=bitcoinrpc
rpcpassword=xxxxxx
rpcallowip=127.0.0.1
testnet=0
txindex=1
walletbroadcast=1
txconfirmtarget=1

Please note that the same behaviour was happening with older releases (from 0.17.x).

Thanks for any help. Please ask for any needed information / testing.
Regards,
{
    std::set<uint256> result;
    AssertLockHeld(cs_wallet);
    std::map<uint256, CWalletTx>::const_iterator it = mapWallet.find(txid);
    if (it == mapWallet.end())
        return result;
    const CWalletTx& wtx = it->second;
    std::pair<TxSpends::const_iterator, TxSpends::const_iterator> range;
    for (const CTxIn& txin : wtx.tx->vin)
    {
        if (mapTxSpends.count(txin.prevout) <= 1)
            continue;  
        range = mapTxSpends.equal_range(txin.prevout);
        for (TxSpends::const_iterator _it = range.first; _it != range.second; ++_it)
            result.insert(_it->second);
    }
    return result;
}
bool CWallet::HasWalletSpend(const uint256& txid) const
{
    AssertLockHeld(cs_wallet);
    auto iter = mapTxSpends.lower_bound(COutPoint(txid, 0));
    return (iter != mapTxSpends.end() && iter->first.hash == txid);
}
void CWallet::Flush(bool shutdown)