Potential bugfix for add_done_callback on AppFutures (#585)
Potential bugfix for add_done_callback on AppFutures (#585)

If a callback was added to an AppFuture before it had a parent assigned,
that callback was silently discarded.

This commit changes that so that the callback behaviour is delegated to
the (Future) superclass - similar to how result() processing is so
delegated.
swift-lang/swift-e-lab,Parsl/parsl,Parsl/parsl,Parsl/parsl,swift-lang/swift-e-lab,Parsl/parsl