Remove repeated retrieval of ValueMap when adapting a model class with multiple ValueMap injections

In an offline discussion with radu.cotescu, he expressed a concern about how if you have a model class (or interface) with a number of ValueMap-based injections it would end up calling Resource.adaptTo(ValueMap.class) multiple times.

While I think this may be primarily an issue for the Resource implementation to solve for, it is true that the effective code executed by Sling Models is going to be less efficient in this case than manual construction of the object.

So I think it is worth exploring.