code: function clear_session(sessionID,req,callback){
      req.sessionStore.destroy(sessionID,function(err){
          if(err){
              logger.error(err);
          }
          logger.debug('session '+sessionID+' destroyed')
          callback && callback(err)
          return null
      })
      return null
  }
docs: clear_session(sessionId,request, callback)
  
  parameters:
  
  sessionId: the session id for the session with the validated client
  
  request: the request object, most likely any request to this web
  server.  It needs to be a request that can see the global
  sessionStore, so if you have multiple servers running
  cas_validate this method might fail (untested in that
  situation).  It is used only to access the global session
  store for the web server, so another option is to pass an
  object {sessionStore:[the relevant session store]}
  
  callback: a function to call when done, will be passed any errors
  
  This method is typically called from clear_st, above.  I am
  exposing it just because it might be useful if developing a
  non-redis-based way of tracking st <-> sessionId key value store.
  
  All it does is destroy the session associated with the passed-in
  sessionId