INT-4237: FWMH: Acquire Lock Before Flushing

JIRA: https://jira.spring.io/browse/INT-4237

It was possible to flush (close) the file while a write was in process; more likely when
`flushWhenIdle` is false.

This probably would not occur in the real world, just tests with short flush intervals,
but certainly possible.

There is already a lock used to prevent concurrent writes while appending; use the same
lock when flushing.

spring-integration-core/src/main/java/org/springframework/integration/util/WhileLockedProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -34,13 +34,16 @@ import org.springframework.messaging.MessagingException;
  *
  */
 public abstract class WhileLockedProcessor {
+
 	private final Object key;
+
 	private final LockRegistry lockRegistry;
 
 	public WhileLockedProcessor(LockRegistry lockRegistry,  Object key) {
 		this.key = key;
 		this.lockRegistry = lockRegistry;
 	}
+
 	public final void doWhileLocked() throws IOException {
 		Lock lock = this.lockRegistry.obtain(this.key);
 		try {
@@ -65,4 +68,5 @@ public abstract class WhileLockedProcessor {
 	 * @throws IOException Any IOException.
 	 */
 	protected abstract void whileLocked() throws IOException;
+
 }