Introduce a simplified PUB-SUB pattern

[S-Gaertner]

The message exchange pattern of the currently defined PUB-SUB is fine, however, the message body is overly complicated. As such PUB-SUB is the only pattern introducing a pattern-specific message body part. Many applications do not need the facilities provided by this pattern body part. For some applications it might even be more natural to integrate the offered behavior (or part of it) into the service description itself or offer dedicated PUB-SUB operations instead of allowing a selection by EntityKeys for example. Additionally, the simplified pattern maps more naturally to some broker-based messaging technologies, for which the subscription matching needed by the traditional PUB-SUB pattern poses a major hurdle.