Bposerow get subscribers of mediator 16141

Pushing change to support GetTopics message that returns subscribers by topic
To make this nicer for Java I’d introduce a proper type to be returned:
~~~scala
@SerialVersionUID(1L) abstract class GetTopics
@SerialVersionUID(1L) case object GetTopics extends GetTopics
def getTopicsInstance: GetTopics = GetTopics
~~~