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/updates.py

@@ -16,7 +16,7 @@ class UpdateMethods(UserMethods):
         try:
             await self.disconnected
         except KeyboardInterrupt:
-            self.disconnect()
+            await self.disconnect()
 
     def run_until_disconnected(self):
         """
@@ -33,7 +33,7 @@ class UpdateMethods(UserMethods):
         try:
             return self.loop.run_until_complete(self.disconnected)
         except KeyboardInterrupt:
-            self.disconnect()
+            self.loop.run_until_complete(self.disconnect())
 
     def on(self, event):
         """