give a expire header that make sure things work
For temp save
1. Remove extra connection to mongodb and using the existing one.
2. remove the console.logs for debug
3. modularized io middle wares to 3 parts: parse cookie, set session object, update session object
4. Remove direct mongodb query for check login status
for temp save
1. Restructured the server start parts
2. Add the initial step for sessionList data structure
Restructure the code and add documentation to the app. The current version passed test