Add a cancellable convenience method

[Ray Tsang]

Add a convenience method to DeferredManager to easily create CancelablePromise. To avoid conflicts in semantics w/ the existing when methods, propose to add cancelable method:

p = dm.cancelable(() -> { ... });
p.cancel();

[Ray Tsang]

/cc @aalmiray

[Ray Tsang]

Maybe this should be part of 1.3 since it will introduce a new signature to the interface. On the other hand, it won't break anyone who didn't implement their own DeferredMananger.

[Ray Tsang]

This is superseded by #126