INT-4441: Fix ACMH Concurrency Problem

JIRA: https://jira.spring.io/browse/INT-4441

Possible concurrent updates to `AbstractCorrelatingMessageHandler.groupIds` and
`expireGroupScheduledFutures`.

(cherry picked from commit f9d5198)

# Conflicts:
#	spring-integration-core/src/main/java/org/springframework/integration/aggregator/AbstractCorrelatingMessageHandler.java

Author: garyrussell

spring-integration-core/src/main/java/org/springframework/integration/aggregator/AbstractCorrelatingMessageHandler.java

@@ -20,10 +20,10 @@
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.Date;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.locks.Lock;
 
@@ -92,7 +92,7 @@ public abstract class AbstractCorrelatingMessageHandler extends AbstractMessageP
 
 	private final Comparator<Message<?>> sequenceNumberComparator = new SequenceNumberComparator();
 
-	private final Map<UUID, ScheduledFuture<?>> expireGroupScheduledFutures = new HashMap<UUID, ScheduledFuture<?>>();
+	private final Map<UUID, ScheduledFuture<?>> expireGroupScheduledFutures = new ConcurrentHashMap<UUID, ScheduledFuture<?>>();
 
 	private final MessageGroupProcessor outputProcessor;