This commit introduces a new service crafted to handle CDC generation
management: listening and reacting to generation changes in the cluster.
The implementation is a stub for now, the service reacts to generation
changes by simply logging the event.
The commit plugs the service in, initializing it in main and test code,
passing a reference to storage_service and having storage_service start
the service (using the `after_join` method): the service only starts
doing its job after the node joins the token ring (either on bootstrap
or restart).
d4937daaea