json_db: exempt keystore from StoredDict conversion

The keystore logic would need to be significantly changed to nicely
interoperate with StoredDict/json_db logic.
(just see KeyStore.__init__() and KeyStore.dump())
For now we exempt the keystore from the recursive StoredDict conversion, as
it is a smaller change that is also easier to review for correctness.

fixes #6066
fixes #6401

also reverts 2d3c2eeea9 (which was an even hackier workaround for #6066)

electrum/wallet_db.py

@@ -1211,6 +1211,14 @@ class WalletDB(JsonDB):
             v = Outpoint(**v)
         return v
 
+    def _should_convert_to_stored_dict(self, key) -> bool:
+        if key == 'keystore':
+            return False
+        multisig_keystore_names = [('x%d/' % i) for i in range(1, 16)]
+        if key in multisig_keystore_names:
+            return False
+        return True
+
     def write(self, storage: 'WalletStorage'):
         with self.lock:
             self._write(storage)