Refactor Transaction and Dataset into DatastoreRequester

Might be missing documentation in a few areas because I wanted to get the structural changes out first before we refine it.

Separates the request logic out from Dataset and Transaction into a `DatastoreRequester` which probably isn't the best name but can be refactored as well. Feel free to send suggestions.
We don't need the doc block here. The `Dataset` constuctor is already aliased to refer to the module.