GossipRouter: additionally listen on IP multicast group and bridge traffic

GossipRouter should be augmented to not just route traffic between TCP endpoints, but also between an IP multicast group and all TCP endpoints.
It should be configured with the properties for a (multicast-based) group X, and then forward messages from X to all of its attached endpoints, and vice versa.
In the future, this concept could be extended to generic bridging functionality, which will allow to construct hierarchical messaging trees, similar to the MBone