Split DRUPAL_BOOTSTRAP_FULL into before and after contrib is loaded

Issue Summary

At present there are a few application where it would be useful to have a fully loaded Drupal system without ANY extra modules being loaded: Plugin Manager for one. (Also, I tend to think that update could benefit from it.) Unfortunately, one does not exist. Currently DRUPAL_BOOTSTRAP_FULL finishes loaded the base system and then loads modules that have been labeled as enabled. This patch adds another stage to the bootstrapping process: DRUPAL_BOOTSTRAP_CORE. It run right before DRUPAL_BOOTSTRAP_FULL. It loads the system up to being fully usable. DRUPAL_BOOTSTRAP_FULL simply finishes the job by loading contrib modules.

This is a necessary step in allowing Drupal to attempt to recover itself in the event that a contrib module is broken / malicious.