Fixed exception when loading images with 'images-versioned' path

> Fixes http://jira.ez.no/browse/EZP-22781
> Status: work in progress

Changes outline: replaces the regular `IOService` in `FieldType\Image\ImageStorage` with a `FieldType\Image\IO\Legacy` one. This one implements the same interface, but dispatches calls between two IOServices: the regular one, operating on 'var/storage/images', and one dedicated to draft images, operating on 'var/storage/images-versioned'.

As a side benefit, makes it possible to simplify ImageStorage, and make it less dependent on specificities from legacy.