Register session_write_close shutdown function

http://php.net/manual/en/function.session-set-save-handler.php:
    When using objects as session save handlers, it is important
    to register the shutdown function with PHP to avoid unexpected
    side-effects from the way PHP internally destroys objects on
    shutdown and may prevent the write and close from being called.
    Typically you should register 'session_write_close' using the
    register_shutdown_function() function.

We're not using objects as session save handlers, but some people
(notably, Synology users) seem to be encountering a similar issue
related to the timing of object destruction. Closing the session
earlier in the shutdown process is a perfectly sane thing to do, so
let's do it.