When using a dispatcher (default or separate cluster dispatcher)
with less than 5 threads the Cluster extension initialization
could deadlock.
It was reproducable by adding a sleep before the Await of GetClusterCoreRef
in the Cluster extension constructor. The reason was that other cluster actors were
started too early and they also tried to get the Cluster extension and thereby blocking
dispatcher threads.
Note that the Cluster extension is started via ClusterActorRefProvider before
ActorSystem.apply returns.
The improvement is to start the cluster child actors lazily when the
GetClusterCoreRef is received.
737a50ebf3