Consistent after save and updateAttributes

-  Ensure consistent behavior in ActiveRecord::afterSave() (fixes #4012)
- changed updateAttributes to be more simple update (fixes #3233)
This is the first case where we name an event class using the event name.
I already raised up your suggestion in my comment, but the comment was gone due to the new commit.

As I explained before, the current design is out of pragmatism.
 If we follow this pattern for all other events, it means we will add 30+ new classes, while the core framework contains only around 300 classes. Is this a big issue? I don't know. Would like to hear other @yiisoft/core-developers opinion on https://github.com/yiisoft/yii2/issues/3882