Revert Starup Session move Below Startup Packages

Revert startup session move below startup packages as this breaks full
page caching in certain ways

- Move `statup/session.php` back above `startup/check_page_cache`
- Change `require_once` back to `require`
Merge pull request #1286 from mkly/revert_dispatcher_move_startup_session

Revert Starup Session move Below Startup Packages