+con #3972 Make Distributedpubsubmediator support consumer group

1. allow Topic have children topics, which are the groups
2. when publish with sendOneMessageToEachGroup flag, it will send to one
actor each group

(cherry picked from commit f421e4260b7871799eded8d0e5d671d3aa5cc109)

Conflicts:
	akka-contrib/src/main/scala/akka/contrib/pattern/DistributedPubSubMediator.scala
	akka-contrib/src/multi-jvm/scala/akka/contrib/pattern/DistributedPubSubMediatorSpec.scala
Merge pull request #15017 from akka/wip-3972-backport-patriknw

+con #3972 Make Distributedpubsubmediator support consumer group (for validation)