title: Node cache doesn't fire the "on expired" callback as intended
body: <p>I'm trying to use node cache's "on expired" event to pull latest data as and when the TTL for the cache key value pair ends. 
  My understanding is it should get called immediately when the cache expires but I think it doesn't gets called until check period ends and the cache cleans up the expired values. Is that so or I'm doing something wrong?</p>
  
  <p><a href="https://www.npmjs.com/package/node-cache#expired" rel="nofollow noreferrer">https://www.npmjs.com/package/node-cache#expired</a></p>