AddressSynchronizer: invalidate balance cache on spv

There was a race incorrectly counting transactions with one
confirmations to the unconfirmed balance instead of the confirmed
balance.
This happened because the balance cache of AddressSynchronizer got
invalidated after `on_event_blockchain_updated` and then again after
`receive_history_callback`->`add_transaction`, however when calling
`AddressSynchronizer.get_balance()` before the tx got spv verified the
height would still be counted as 0 (unconfirmed), populating the balance
cache again with the unconfirmed balance.
I noticed this only on QML due to timing differences to Qt.
Invalidating the cache in `AddressSynchronizer.add_verified_tx()` after
the tx got verified causes the balance to get recalculated and shown
correctly.

electrum/address_synchronizer.py

@@ -652,6 +652,7 @@ class AddressSynchronizer(Logger, EventListener):
         with self.lock:
             self.unverified_tx.pop(tx_hash, None)
             self.db.add_verified_tx(tx_hash, info)
+            self.invalidate_cache()
         util.trigger_callback('adb_added_verified_tx', self, tx_hash)
 
     @with_lock