Move get_dict method from wallet_db to json_db.

Define overloaded methods: _convert_dict _convert_values

electrum/wallet_db.py

@@ -1337,14 +1337,6 @@ class WalletDB(JsonDB):
         assert isinstance(txid, str)
         self.tx_fees.pop(txid, None)
 
-    @locked
-    def get_dict(self, name) -> dict:
-        # Warning: interacts un-intuitively with 'put': certain parts
-        # of 'data' will have pointers saved as separate variables.
-        if name not in self.data:
-            self.data[name] = {}
-        return self.data[name]
-
     @locked
     def num_change_addresses(self) -> int:
         return len(self.change_addresses)