Update session component to not specify a default element.

By omitting the default element name backwards compatibility is
restored.
Merge pull request #3608 from cakephp/3.0-session-component-fix

Update session component to not specify a default element.
Login field will now get a uniqueness validator
Merge pull request #3610 from lorenzo/3.0

Login field will now get a uniqueness validator
Remove backwards compatibility feature.

It's not necessary to check for .ctp extension as fallback if alternate
extension is set.
Remove redundant method arg.

Getting rid of $exit param from callback since Controller::redirect() no longer exits.
Merge pull request #3619 from ADmad/3.0-redirect-exit

Remove redundant method arg.
Merge pull request #3616 from ADmad/3.0-view-ext

Remove backwards compatibility feature.
Refactor _eventManager accessors to Cake\Event\EventManagerTrait

Addresses issue #3592
Use a single method to set and get EventManager.
Always returns an EventManager, if none was previously set.
Cake\Utility\ObjectRegistry is aware of an optional EventManager.
Each class extending ObjectRegistry will need to use the trait.

Explicitly assign and access EventManager via protected field

This avoids issues with unit tests that mock the whole class,
causing eventManager() to always return null;