Rename JobResult to JobOutput

[plastikfan]

The problem with using the name JobResult is that it's difficult for client code to instantiate it with a sensible name. Eg in extendio, we already have the abstraction called TraverseResult so if we try to create a related type we end up with something like:

TraverseResultJobResult

Which doesn't feel right. If we rename JobResult to JobOutput then we can rename the above to be

TraverseResultJobOutput

Which is a vast improvement.

So the worker pool should never use the term Result in it type names as this is such a natural term for the client to use.