Page cache stack middleware

The procedural functions related to the internal page cache in bootstrap.inc, common.inc and DrupalKernel have been converted into a PageCache stack middleware. This allows contrib and custom modules to completely swap out the internal page cache implementation or even remove it entirely.

• drupal_page_cache_get_cid()
• drupal_page_get_cache()
• drupal_page_set_cache()
• drupal_serve_page_from_cache()
• DrupalKernel::handlePageCache()

These functions are not commonly used by contrib / custom modules. Notable exceptions are a) modules wishing to swap out the internal page cache like Boost and Authcache and b) modules which (ab-)use the internal page cache for their own purposes like e.g. Administration menu. The former kind of modules should simply ship their own implementation/subclass of the middleware while the latter can be fixed by either using their own cache bin or supplying their own page cache policy rule sets.