Decouple the building of Models per bucket from fetching them

[micahr]

It would be very helpful to provide a mechanism to gets all the models that are in a "bucket" without having them automatically fetched.

A proposal would be to change the signature of buildModelsWrapper in such a way that it will accept a function to call in the forEachParallel call and default to the current one if it's not passed in. This will allow developers to handle fetching of models or any other customizations that need to be done without re-implementing the entire process.