Client part of GMS. Whenever a new member wants to join a group, it starts in the CLIENT role.
No multicasts to the group will be received and processed until the member has been joined and
turned into a SERVER (either coordinator or participant, mostly just participant). This class
Join (called by clients who want to join a certain group, and
ViewChange which is called by the coordinator that was contacted by this client, to
tell the client what its initial membership is.