code: function clear_st(st,req,callback){
      redclient.get(st
                   ,function(err,sid){
                        if(!err){
                            redclient.del(st);
                            if(sid){
                                logger.debug('got '+sid+' from redis')
                                return clear_session(sid,req,callback)
                            }else{
                                logger.debug('got '+sid+' from redis, nothing to clear')
                                callback && callback()
                            }
                        }else{
                            callback && callback(err)
                        }
                        return null
                    })
      return null
  }
docs: clear_st(st,request, callback)
  
  parameters:
  
  st: the session ticket, parsed from the CAS POST message
  
  request: the request object, which is needed for its link to the
  general session store for the web server
  
  callback: a function to call, will be passed any errors
  
  look in the redis db for the session associated with the given
  session ticket (the st), and then remove the session from the
  session store