Commerce product reference support. Fixed debug message not showing without accurate tracking turned on

Making sure that hook_exit is always run, also if changes are done to the system through cron or other means by registering our hook_exit as a shutdown callback. Fixes #2188331.

Refactoring of page tracking to use cache tables (thus support for redis, memcache, etc). Swapped to use lighter loading of entities

Get rid oh php notice when an entity reference field uses a view to select targets instead of selected bundles.