On initial cluster join, other members should be notified

[guusdk]

With the changes for #65, local nodes no longer depend on Hazelcast's memberAdded event to trigger org.jivesoftware.openfire.cluster.ClusterEventListener#joinedCluster(byte[]). Instead, a new cluster task (NewClusterMemberJoinedTask) is started by the joining node to trigger these.

This task should also be executed when the local node joins a pre-existing cluster, which it currently doesn't yet do.