Changes to JEP 453

[Attila Kelemen]

> In the JEP, the state CANCELLED has disappeared from the possible state of Subtask ?
> I'm puzzled by this change.
>

I'm puzzled that you are puzzled :) Because you said: "I propose to
simplify the design and remove the state CANCELLED". With which I
wholeheartedly agree.

In my opinion, this is a much cleaner design, because it leaves more
options for the STS policy to decide what is actually meant by being
cancelled. Also, conceptually simpler, because now the state of
`Subtask` represents what is actually happening rather than what was
wished from it (by a shutdown call. Also, it is of little concern to
the client of STS, because it shouldn't really based its decision on
that (since that decision should be centralized in the STS policy).