CollectionTransformer performance

[kontoulis]

Description
Resolves the performance issue #1.

After debugging and measuring execution times in several scenarios, it turns out that the performance issue is related to the number of translations passed to ModelTransformer@applyTranslations.
For example if we have a collection of 1000 objects to be translated and each one of them has 5 translated properties,
ModelTransformer@applyTranslations has to loop over these 5000 translations for every one of those 1000 objects.
This gets even worse when we perform deep translation.

Solution
Passing only the required translations to ModelTransformer@applyTranslations resolves this issue.
Required translations are the model translations and the relations translations if we have to perform deep translation.

[kirkbushell]

It looks good, but there's no tests, and there's a substantial amount of logic and complexity in the request.

[kontoulis]

@kirkbushell I have changed only implementation details and the functionality is already covered in EndToEndTest. I am not sure what else to test.

Edit: It actually ended up with some complexity in order to pass the tests, as when I started it I didn't consider all the use cases. This was basically TDD, just with existing tests :P