INT-3346: Add @Id to CMDbMS.MessageDocument

JIRA: https://jira.spring.io/browse/INT-3346

An `@Id` field is required when MongoDB auditing
is enabled in the ApplicationContext.

This change does not make the document Auditable,
it just prevents an exception being thrown when
auditing is enabled.

spring-integration-mongodb/src/test/java/org/springframework/integration/mongodb/store/ConfigurableMongoDbMessageGroupStoreTests.java

@@ -81,7 +81,7 @@ public class ConfigurableMongoDbMessageGroupStoreTests extends AbstractMongoDbMe
 		TestGateway gateway = context.getBean(TestGateway.class);
 		String result = gateway.service("foo");
 		assertEquals("FOO", result);
-
+		context.close();
 	}
 
 	public static interface TestGateway {