INT-3616: Zookeeper LockRegistry

JIRA: https://jira.spring.io/browse/INT-3616

Zookeeper implementation of `LockRegistry`, for example to support
clustered aggregators (when using external message group stores).

INT-3616: Polishing; PR Comments

spring-integration-core/src/main/java/org/springframework/integration/metadata/ListenableMetadataStore.java

@@ -22,6 +22,7 @@ package org.springframework.integration.metadata;
  * invoked when changes occur in the metadata store.
  *
  * @author Marius Bogoevici
+ * @since 4.2
  */
 public interface ListenableMetadataStore extends ConcurrentMetadataStore {
 

spring-integration-core/src/main/java/org/springframework/integration/metadata/MetadataStoreListener.java

@@ -20,6 +20,7 @@ package org.springframework.integration.metadata;
  * A callback to be invoked whenever a value changes in the data store.
  *
  * @author Marius Bogoevici
+ * @since 4.2
  */
 public interface MetadataStoreListener {
 

spring-integration-core/src/main/java/org/springframework/integration/support/locks/ExpirableLockRegistry.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.integration.support.locks;
+
+/**
+ * A {@link LockRegistry} implementing this interface supports the removal of aged locks
+ * that are not currently locked.
+ *
+ * @author Gary Russell
+ * @since 4.2
+ *
+ */
+public interface ExpirableLockRegistry extends LockRegistry {
+
+	/**
+	 * Remove locks last acquired more than 'age' ago that are not currently locked.
+	 * @param age the time since the lock was last obtained.
+	 * @throws IllegalStateException if the registry configuration does not support this feature.
+	 */
+	void expireUnusedOlderThan(long age);
+
+}