public void receive (String aRoutingKey, Receiver aReceiver) {
    List<Receiver> list = listeners.get(aRoutingKey);
    if(list == null) {
      list = new ArrayList<>();
      listeners.put(aRoutingKey, list);
    }
    list.add(aReceiver);
  }
a simple, non-thread-safe implementation of the 
{@link MessageBroker} interface. Useful for testing.

@author Arik Cohen
@since Jul 10, 2016
src/main/java/com/creactiviti/piper/core/messagebroker/SyncMessageBroker.java