Revert "fix the Flash middleware loading the session on every request (very dangerous especially with Rack::Cache), it should only be loaded when the flash method is called"

This reverts commits e3069c64b2c5ddc7a5789b55b8efd4902d9e9729 and 2b2983d76fd11efc219273036a612f47cfaa5bfa.

Reason: This add a non-backward compatible change in the way that flash
works now (swept in every request).