* moved to cluster tests, in new package akka.cluster.testkit
* changed config in tests
* migration guide
* documentation clarificiations for Downing and Leaving
* update warnings in Singleton and Sharding
* While waiting for update to comple it will now deliver messages to other
already started entities immediately, instead of stashing
* Unstash one message at a time, instead of unstashAll
* Append messageBuffer for messages to the entity that we are waiting for,
instead of stashing
* Test to confirm the improvements
* Fixing a few other missing things
* receiveStartEntity should process the change before starting the entity
* lastMessageTimestamp should be touched from overridden deliverTo
* handle StoreFailure
