De-duplicate session flash features.

With a proper FlashComponent and FlashHelper we don't need to have
a duplicated set of features in the Session object. This change also
unifies the keys SessionComponent/SessionHelper and
FlashComponent/FlashHelper use so no matter which interface you use,
everything plays nicely together.

Refs cakephp/docs#1430
Merge pull request #3750 from markstory/de-dupe-flash

De-duplicate session flash features.