<!-- 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,
{
    LOCK(cs_wallets);
    auto it = g_load_wallet_fns.emplace(g_load_wallet_fns.end(), std::move(load_wallet));
    return interfaces::MakeHandler([it] { LOCK(cs_wallets); g_load_wallet_fns.erase(it); });
}
static Mutex g_wallet_release_mutex;
static std::condition_variable g_wallet_release_cv;
static std::set<std::string> g_unloading_wallet_set;
static void ReleaseWallet(CWallet* wallet)
{
    const std::string name = wallet->GetName();
    wallet->WalletLogPrintf("Releasing wallet\n");
    wallet->Flush();
    delete wallet;
    
    {
        LOCK(g_wallet_release_mutex);
        if (g_unloading_wallet_set.erase(name) == 0) {
            
            return;
        }
    }
    g_wallet_release_cv.notify_all();
}
void UnloadWallet(std::shared_ptr<CWallet>&& wallet)