wallet: make labels private, and access to need lock

e.g. labels plugin iterated over wallet.labels on asyncio thread while user could trigger an edit from Qt thread

electrum/wallet_db.py

@@ -1051,7 +1051,7 @@ class WalletDB(JsonDB):
         self.tx_fees.pop(txid, None)
 
     @locked
-    def get_dict(self, name):
+    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: