KBGH-356: Fix thread naming with no bean name

See spring-attic/spring-cloud-stream-binder-kafka#356

Use token 'consumer' for thread prefix when no bean name.

**cherry-pick to 2.0.x, 1.3.x**

Author: garyrussell

Diff for: spring-kafka/src/main/java/org/springframework/kafka/listener/ConcurrentMessageListenerContainer.java

@@ -153,9 +153,8 @@ protected void doStart() {
 					container = new KafkaMessageListenerContainer<>(this, this.consumerFactory,
 							containerProperties, partitionSubset(containerProperties, i));
 				}
-				if (getBeanName() != null) {
-					container.setBeanName(getBeanName() + "-" + i);
-				}
+				String beanName = getBeanName();
+				container.setBeanName((beanName != null ? beanName : "consumer") + "-" + i);
 				if (getApplicationEventPublisher() != null) {
 					container.setApplicationEventPublisher(getApplicationEventPublisher());
 				}