backend class back to an array

this started in #595930: from github to d.o cvs(aka start 6.x-2.0-dev and get access)

quoting sdboyer:

- At the moment, our backend classes are really little more than arrays. There's no substantial reason for us to have made them objects at all. The drupal OO philosohy I adhere to says, "unless there's a clear reason to make it an object, don't." That applies even if our explicit goal is to OO-ify something in drupal-land.

Backend class can be an array now. At start I though backend class was going to handle the optional functionality, but then each entity as a class make this non-real, so IIRC we can change it with no so much effort.