wallet: cache more in get_tx_fee

closes #4879

electrum/address_synchronizer.py

@@ -717,11 +717,14 @@ class AddressSynchronizer(PrintError):
             return None
         if hasattr(tx, '_cached_fee'):
             return tx._cached_fee
+        with self.lock, self.transaction_lock:
             is_relevant, is_mine, v, fee = self.get_wallet_delta(tx)
             if fee is None:
                 txid = tx.txid()
                 fee = self.tx_fees.get(txid)
-        if fee is not None:
+            # cache fees. if wallet is synced, cache all;
+            # otherwise only cache non-None, as None can still change while syncing
+            if self.up_to_date or fee is not None:
                 tx._cached_fee = fee
         return fee