oc"><p>Create a subscriber that works in a background thread 
   and updates an MVar, whenever new data are available;
   the function is in fact a special case of <code><a href="Network-Mom-Stompl-Patterns-Basic.html#v:withSubThread">withSubThread</a></code>,
   where the application callback updates an MVar.
   Note that the MVar must not be empty when the function
   is called, otherwise, it will block on modifying the MVar.
</p><ul><li> <code>Con</code>: Connection to a Stomp broker;
</li><li> String: S