OPEN - issue INT-1063: MessageStore: correlation and grouping API

Fix stupid concurrent modification exception

org.springframework.integration/src/main/java/org/springframework/integration/aggregator/CorrelatingMessageHandler.java

@@ -318,7 +318,7 @@ public class CorrelatingMessageHandler extends AbstractMessageHandler implements
 							}
 							for (Message<?> message : all) {
 								discardChannel.send(message);
-								store.delete(message.getHeaders().getId());
+								store.delete(key, message.getHeaders().getId());
 							}
 						}
 					}

org.springframework.integration/src/main/java/org/springframework/integration/store/SimpleMessageStore.java

@@ -98,7 +98,7 @@ public class SimpleMessageStore implements MessageStore {
 		if (collection==null) {
 			return Collections.emptySet();
 		}
-		return Collections.unmodifiableCollection(collection);
+		return new HashSet<Message<?>>(collection);
 	}
 
 	public void put(Object correlationId, Collection<Message<?>> messages) {