public boolean pending() {
        return current < wanted;
    }
An immutable generation, with wanted and current generation fields.  Wanted generation
is increased when an action (restart services or reboot are the available
actions) is wanted, current is updated when the action has been done on the node.

@author hmusum
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/node/Generation.java