ActiveMQSpanConsumer init() {
    if (result == null) {
      synchronized (this) {
        if (result == null) {
          result = doInit();
        }
      }
    }
    return result;
  }
Lazy creates a connection and registers a message listener up to the specified concurrency level.
This listener will also receive health notifications.
zipkin-collector/activemq/src/main/java/zipkin2/collector/activemq/LazyInit.java