Change the way no_updates mode is enabled

See discussion on https://github.com/LonamiWebs/Telethon/commit/49713b2.

The problem with the automatic approach is that some scripts may do
some "fancier" things with the way they register updates, so it was
prone to failure (a handler could be added but since the last request
was without updates, nothing would be received).

This new approach is a bit more annoying to opt-into but also more
explicit.

telethon/client/updates.py

@@ -32,6 +32,17 @@ class UpdateMethods:
         finally:
             await self.disconnect()
 
+    async def set_receive_updates(self: 'TelegramClient', receive_updates):
+        """
+        Change the value of `receive_updates`.
+
+        This is an `async` method, because in order for Telegram to start
+        sending updates again, a request must be made.
+        """
+        self._no_updates = not receive_updates
+        if receive_updates:
+            await self(functions.updates.GetStateRequest())
+
     def run_until_disconnected(self: 'TelegramClient'):
         """
         Runs the event loop until the library is disconnected.