ChoosableSet.choose() may return invalid registration information

[hedyn]

When an event is sent to the restarted node, the ServerID is obtained before the reboot.
"iter" should be set to null after calling the add, remove, merge methods of ChoosableSet class.

io.vertx.spi.cluster.zookeeper.impl.ChoosableSet:

public void add(T elem) {
ids.add(elem);
iter = null;
}

public void remove(T elem) {
ids.remove(elem);
iter = null;
}

public void merge(ChoosableSet<T> toMerge) {
ids.addAll(toMerge.ids);
iter = null;
}