Refactored AR find
Added `isAssociative()` and `isIndexed()` to `yii\helpers\ArrayHelper`
Finished refactoring find() .
Merge branch 'master' into split-find-into-find-findOne-findAll
updated change log.
Merge pull request #3003 from yiisoft/split-find-into-find-findOne-findAll

Refactored AR find to fix #2999