Streamline drupal_bootstrap() and expose the flow to code profilers

Spin-off from #615822-37: Benchmarking / profiling of D7:

This patch simply

1) moves the switch from _drupal_bootstrap() into drupal_bootstrap()

2) moves the code in larger switch cases into dedicated functions

The main purpose is to see the actual performance of every bootstrap phase. It was considered as a temporary patch, but I seriously don't see why it shouldn't be permanent. In fact, the flow of drupal_bootstrap() is much more grokable with this patch.

There is only one nitpick that is debatable: _drupal_bootstrap() had a static $cache; variable that was used both for DRUPAL_BOOTSTRAP_PAGE_CACHE and DRUPAL_BOOTSTRAP_PAGE_HEADER to figure out whether we need to send a X-Drupal-Cache: MISS HTTP header.

For now, I replaced that with a drupal_static(). An alternative would be to use a global variable. Yet another alternative would be to perhaps stuff that logic into drupal_page_is_cacheable().