fixes #2650

Fixes #2955: Changed the signature of ActiveQuery constructors and `ActiveRecord::createQuery()` to simplify customizing ActiveQuery classes

fixes #2957

fix for https://github.com/yiisoft/yii2/pull/2958

Refactored ActiveRecord::find() so that find(null) will also return an ActiveRecord instance instead of ActiveQuery.

issue #2892

so information about changed values is available in `afterSave`-event.

fixes #2892

this problem occurs when joining active record but not selecting the
primary key field. records get populated but they do not have primary
key value populated which results in only one record left over in the
result.

issue #2852