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/client/downloads.py

@@ -281,7 +281,7 @@ class DownloadMethods(UserMethods):
             if exported:
                 await self._return_exported_sender(sender)
             elif sender != self._sender:
-                sender.disconnect()
+                await sender.disconnect()
             if isinstance(file, str) or in_memory:
                 f.close()