make batch write "all or nothing"

Currently, the batch write mode is nothing but a grammar sugar, not really taking advantage of any adapter's batch write method.

To ensure errors are properly handled, one still need to write array iterations in the callback. If you want to retry creation with existing data, you'd have to remove successfully created objects (otherwise duplicates happen).

The currently behavior `callback([, Error, ], [Object, null, Object])` breaks "all or nothing" principle for atomicity, make it difficult to trace Error (you have to print an array, or do iteration), and breaks API consistency (image you accidentally passed an Array to Model.create when you need it to be an object).

To avoid confusions and headaches, this mode should either be 1). removed, as it's not really "bulk inserting", 2). made atomic (at lease validate them all before starting to `save()`).