External sink activity progress serialization/deserialization mismatch

Ancient bug (since inception of externalsink activity).
I think it would probably only cause a spurious thor manager log warning, e.g.:

 "Failed to find activity, during progress deserialization, id=0 - graph(graph1, 1)"

The slave sink activity was incorrectly initializing itself with a output edge, which caused it to be serialized (always with 0). The master would then see this as the beginning of an additional activity after the sink , but fail to find a matching activity id (0), and result in the above message.