Add kernel.terminate event

The only kernel event we use right now is kernel.request, which is used for "Drupal is initializing". Here is a patch to add the kernel.terminate event, for "Drupal is terminating". This event is generated after the response is sent but before PHP is shutdown. I don't think it's extremely useful, but there's no alternative to do things that had to be done in hook_exit() in D7 (hook_exist() was removed from D8). This is a super light weight feature, with a test.