wallet.is_up_to_date: fix flickering during sync due to race

AddressSynchronizer.add_address called synchronizer.add, which would only
schedule adding the addr to the Synchronizer in the next event loop iter.
If during that time, the synchronizer called adb.set_up_to_date(True),
the wallet would falsely believe and advertise itself as up_to_date
(as the wallet would see wallet.synchronize not creating new addresses,
and adb (via synchronizer) telling it is up_to_date).
Moments later, the synchronizer._add_address is finally executed and
up_to_date=False propagates out synchronizer->adb->wallet.

electrum/scripts/watch_address.py

@@ -32,12 +32,12 @@ class Notifier(SynchronizerBase):
     async def main(self):
         # resend existing subscriptions if we were restarted
         for addr in self.watched_addresses:
-            await self._add_address(addr)
+            self.add_address(addr)
         # main loop
         while True:
             addr = await self.watch_queue.get()
             self.watched_addresses.add(addr)
-            await self._add_address(addr)
+            self.add_address(addr)
 
     async def _on_address_status(self, addr, status):
         print_msg(f"addr {addr}, status {status}")