* 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
* Implement the join to seed nodes process
When a new node is started started it sends a message to all
seed nodes and then sends join command to the one that answers
first.
* Configuration of seed-nodes and auto-join
* New JoinSeedNodeSpec that verifies the auto join to seed nodes
* In tests seed nodes are configured by overriding seedNodes
function, since addresses are not known before start
* Deputy nodes are the live members of the seed nodes (not sure if
that will be the final solution, see ticket 2252
* Updated cluster.rst with latest info about deputy and seed nodes
