Revert "Take into account time spent in AR even if a redirect occurs or if it is after the render" because it's causing test failures about missing db_during_render.

This reverts commit 60b55bac.

Previously the runtime was reset implicitly when #cleanup_view_runtime was called at the end of most requests. However, this doesn't happen when the request redirects, or send_file is called.  Consequently, the ActiveRecord runtime recorded in the logs included the time taken for both the current request and the previous redirect.  Explicitly resetting at the start of each request ensures that this can't happen, no matter what occurs previously.

* AS::Notifications#instrument should not measure anything, it is not its responsibility;

* Adding another argument to AS::Notifications#instrument API needs to be properly discussed;

Fix up AR extension by using Railties module instead of Rails and requiring ControllerRuntime at initialization time instead of boot