wallet: bugfix: ensure atomicity in settings updates

- Settings updates were not thread-safe, as they were executed in
  three separate steps:

  1) Obtain settings value while acquiring the settings lock.
  2) Modify settings value.
  3) Overwrite settings value while acquiring the settings lock.

  This approach allowed concurrent threads to modify the same base value
  simultaneously, leading to data loss. When this occurred, the final
  settings state would only reflect the changes from the last thread
  that completed the operation, overwriting updates from other threads.

  Fix this by making the settings update operation atomic.

- Add test coverage for this behavior.

Co-authored-by: furszy <matiasfurszyfer@protonmail.com>

src/wallet/load.cpp

@@ -69,7 +69,7 @@ bool VerifyWallets(WalletContext& context)
             // Pass write=false because no need to write file and probably
             // better not to. If unnamed wallet needs to be added next startup
             // and the setting is empty, this code will just run again.
-            chain.updateRwSetting("wallet", wallets, /* write= */ false);
+            chain.overwriteRwSetting("wallet", wallets, /*write=*/false);
         }
     }