Treat Configuration as a mutable object, not immutable

[johnynek]

In the scalding code, we are treating the Configuration like an immutable object and passing functions from Configuration => Configuration around, the problem is, Configuration is mutable, and it is not at all clear how to reason about such changes.

I think the better approach is to use the Reader Monad, which is basically combinators on (Configuration) => T and T => ((Configuration) => U)

[johnynek]

Fixing #529 is related here. Using scalding.Execution is probably the way forward.