Save pts and date in a tuple for immutability

This way it is easy and cheap to copy the two required values
to all incoming updates in case we need to getDifference since
the previous pts/date to fetch entities.

This is still a work in progress.

telethon/network/mtprotosender.py

@@ -547,7 +547,7 @@ class MTProtoSender:
         self._log.debug('Handling update {}'
                         .format(message.obj.__class__.__name__))
         if self._update_callback:
-            await self._update_callback(message.obj)
+            self._update_callback(message.obj)
 
     async def _handle_pong(self, message):
         """