* The problem was that we didn't wait for the testconductor.shutdown Future
to complete and therefore barriers could be triggered in unexpected order.
The reason why we didn't await, was that during shutdown the Future was
completed with client disconnected failure. I have fixed that and added
await to all shutdowns.
* Major refactoring to remove the need to use special
Cluster instance for testing. Use default Cluster
extension instead. Most of it is trivial changes.
* Used failure-detector.implementation-class from config
to swap to Puppet
* Removed FailureDetectorStrategy, since it doesn't add any value
* Added Cluster.joinSeedNodes to be able to test seedNodes when Addresses
are unknown before startup time.
* Removed ClusterEnvironment that was passed around among the actors,
instead they use the ordinary Cluster extension.
* Overall much cleaner design
* self is initially not member (in gossip state)
* if the join to seed nodes timeout it joins itself, and becomes
singleton cluster
* remove the special case handling of singelton cluster in gossip
merge, since singleton cluster is not the normal state when joining
any more
