Skip bootstrap.php.cache/classes cache in debug mode

> http://jira.ez.no/browse/https://jira.ez.no/browse/EZP-22590

`bootstrap.php.cache` and classes cache can really get in the way with interactive debugging sessions.

This changes the front controller so that when debugging, if the \ XDEBUG_SESSION` cookie is set, the cache files above won't be loaded, so that the actual, original files can be stepped through.

### Points to sort out
- [x] What about other debugging methods / extensions ? Different cookie ? _We skip cache in debug mode, no debugger condition_
- [x] Does it cover all scenarios (command line / web). _Yep, no conditions on cookie._
- [x] Create story, if acceptable