GH-3166: SMLC: synchronize on processorThreadsToInterrupt

Fixes: #3166

The `processorThreadsToInterrupt` is iterated in the `shutdownAndWaitOrCallback()`,
and apparently one of the processors has finished successfully removing itself from the `processorThreadsToInterrupt` list.
This leads to the `ConcurrentModificationException` on the mentioned above iteration.

* Fix `SimpleMessageListenerContainer` making the `processorThreadsToInterrupt` as a `Collections.synchronizedList()`
* Wrap `processorThreadsToInterrupt` iteration in the `shutdownAndWaitOrCallback()` with a `synchronized (this.processorThreadsToInterrupt)`

**Auto-cherry-pick to `3.2.x`**

Author: artembilan

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/listener/SimpleMessageListenerContainer.java

@@ -19,6 +19,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
@@ -115,7 +116,7 @@ public class SimpleMessageListenerContainer extends AbstractMessageListenerConta
 
 	private final ActiveObjectCounter<BlockingQueueConsumer> cancellationLock = new ActiveObjectCounter<>();
 
-	private final List<Thread> processorThreadsToInterrupt = new ArrayList<>();
+	private final List<Thread> processorThreadsToInterrupt = Collections.synchronizedList(new ArrayList<>());
 
 	private long startConsumerMinInterval = DEFAULT_START_CONSUMER_MIN_INTERVAL;
 
@@ -706,7 +707,9 @@ protected void shutdownAndWaitOrCallback(@Nullable Runnable callback) {
 				else {
 					logger.info("Workers not finished.");
 					if (isForceCloseChannel() || this.stopNow.get()) {
-						this.processorThreadsToInterrupt.forEach(Thread::interrupt);
+						synchronized (this.processorThreadsToInterrupt) {
+							this.processorThreadsToInterrupt.forEach(Thread::interrupt);
+						}
 						canceledConsumers.forEach(consumer -> {
 							if (logger.isWarnEnabled()) {
 								logger.warn("Closing channel for unresponsive consumer: " + consumer);