Revert disconnect() to be async again (#1133)

It's the only way to properly clean all background tasks,
which the library makes heavy use for in MTProto/Connection
send and receive loops.

Some parts of the code even relied on the fact that it was
asynchronous (it used to return a future so you could await
it and not be breaking).

It's automatically syncified to reduce the damage of being
a breaking change.

telethon_examples/gui.py

@@ -365,7 +365,7 @@ async def main(loop, interval=0.05):
         if 'application has been destroyed' not in e.args[0]:
             raise
     finally:
-        app.cl.disconnect()
+        await app.cl.disconnect()
 
 
 if __name__ == "__main__":