+clu #3627 Cluster router group  with multiple paths per node

* Use the ordinary routees.paths config property instead of
  cluster.routees-path
* Backwards compatible in deprecation phase
+act #3663 Package BalancingDispatcher for usage in router pool

* In fact, make it easy to define any dedicated dispatcher for a pool
+con #3669 Possibility to define routing logic in DistributedPubSub

* Serialization check in contrib
Merge pull request #1772 from akka/wip-3627-cluster-routee-paths-patriknw

+clu #3627 Cluster router group  with multiple paths per node
Merge pull request #1776 from akka/wip-3663-balancing-disp-patriknw

+act #3663 Package BalancingDispatcher for usage in router pool
Merge pull request #1778 from akka/wip-3669-routing-in-pubsub-patriknw

+con #3669 Possibility to define routing logic in DistributedPubSub
Merge remote-tracking branch: upstream/master