Tests for memory cycles in App and Runner.

Depends on Test::Memory::Cycle.
weak_ref 'app' attribute in Role::Handler.

Also clean up how the Handlers are created in Core::App, passing in app
as an item instead of adding it to a config has that was creating another
memory cycle.
Remove memory cycles involving app hooks.

Close over a weakened copy of app object when creating the 'after' hook
that flushes sessions.

Resolves #610.
Merge branch 'bugfix/circular_refs'