* 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
