title: What's exactly the meaning of "saveUninitialized","resave" and "rolling" properties in express-session?
body: <p>Recently，I'm learning about the middleware "express-session" of express, I want to understand all properties in the given options.But when I read about the API of <code>express-session</code>, I'm confused with the three </p>
  
  <p>properties:<code>saveUninitialized</code>, <code>resave</code> and <code>rolling</code>.</p>
  
  <p>They all have an effect on cookie setting or session operation, so what's the difference and connection of them?</p>
  
  <p>Hope somebody can help me to distinguish them, </p>
  
  <p>Thanks a lot! </p>