move original bootstrap routines back into bootstrap class out of bootstrap listener.
improve code comments in bootstrap listener.
introduce PseudoKernel.

PseudoKernel allows Drupal's bootstrap to be treated as a service,
which means it can be injected into dependencies so that Drupal
can be only bootstrapped as needed when the request is not a Drupal
route.

Bootstrap now depends on the event dispatcher so that events can
be triggered before and after bootstrap phases.
additional check for drupal routes.
merge up.
Bootstrap listener request events only on master request.
code comment.
lower priority of drupal route loader.