INT-4560: Fix Race in FileSystemPersistentAOFLF

JIRA: https://jira.spring.io/browse/INT-4560

Reproduced and tested with

```java
@SpringBootApplication
public class So53521593Application {

	private static final Logger logger = LoggerFactory.getLogger(So53521593Application.class);

	public static void main(String[] args) {
		SpringApplication.run(So53521593Application.class, args);
	}

	@Bean
	public IntegrationFlow flow() {
		ExecutorService exec = Executors.newFixedThreadPool(10);
		return IntegrationFlows.from(Files.inboundAdapter(new File("/tmp/foo")).filter(
					new MyFilter(new SimpleMetadataStore(), "foo")),
						e -> e.poller(Pollers.fixedDelay(5, TimeUnit.SECONDS)
								.maxMessagesPerPoll(10)))
				.channel(MessageChannels.executor(exec))
				.<File>handle((p, h) -> {
					try {
						p.delete();
						logger.info(p.toString());
						Thread.sleep(10_000);
					}
					catch (InterruptedException e1) {
						Thread.currentThread().interrupt();
					}
					return null;
				})
				.get();
	}
}

class MyFilter extends FileSystemPersistentAcceptOnceFileListFilter {

	public MyFilter(ConcurrentMetadataStore store, String prefix) {
		super(store, prefix);
	}

	@Override
	protected long modified(File file) {
		long modified = super.modified(file);
		System.out.println(modified);
		return modified;
	}

}
```

**cherry-pick to 5.0.x, 4.3.x**

spring-integration-file/src/main/java/org/springframework/integration/file/filters/AbstractPersistentAcceptOnceFileListFilter.java

@@ -77,17 +77,27 @@ public abstract class AbstractPersistentAcceptOnceFileListFilter<F> extends Abst
 			String oldValue = this.store.putIfAbsent(key, newValue);
 			if (oldValue == null) { // not in store
 				flushIfNeeded();
-				return true;
+				return fileStillExists(file);
 			}
 			// same value in store
 			if (!isEqual(file, oldValue) && this.store.replace(key, oldValue, newValue)) {
 				flushIfNeeded();
-				return true;
+				return fileStillExists(file);
 			}
 			return false;
 		}
 	}
 
+	/**
+	 * Check if the file still exists; default implementation returns true.
+	 * @param file the file.
+	 * @return true if the filter should return true.
+	 * @since 4.3.19
+	 */
+	protected boolean fileStillExists(F file) {
+		return true;
+	}
+
 	/**
 	 * {@inheritDoc}
 	 * @since 4.0.4

spring-integration-file/src/main/java/org/springframework/integration/file/filters/FileSystemPersistentAcceptOnceFileListFilter.java

@@ -42,4 +42,15 @@ public class FileSystemPersistentAcceptOnceFileListFilter extends AbstractPersis
 		return file.getAbsolutePath();
 	}
 
+	/**
+	 * Check that the file still exists, to avoid a race condition when multi-threaded and
+	 * another thread removed the file while we were waiting for the lock.
+	 * @since 4.3.19
+	 */
+	@Override
+	protected boolean fileStillExists(File file) {
+		return file.exists();
+	}
+
+
 }

spring-integration-file/src/test/java/org/springframework/integration/file/filters/PersistentAcceptOnceFileListFilterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2015 the original author or authors.
+ * Copyright 2013-2018 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.
@@ -126,6 +126,11 @@ public class PersistentAcceptOnceFileListFilterTests extends AcceptOnceFileListF
 				new SimpleMetadataStore(), "rollback:");
 		File[] files = new File[] {new File("foo"), new File("bar"), new File("baz")};
 		List<File> passed = filter.filterFiles(files);
+		assertEquals(0, passed.size());
+		for (File file : files) {
+			file.createNewFile();
+		}
+		passed = filter.filterFiles(files);
 		assertTrue(Arrays.equals(files, passed.toArray()));
 		List<File> now = filter.filterFiles(files);
 		assertEquals(0, now.size());
@@ -137,6 +142,9 @@ public class PersistentAcceptOnceFileListFilterTests extends AcceptOnceFileListF
 		now = filter.filterFiles(files);
 		assertEquals(0, now.size());
 		filter.close();
+		for (File file : files) {
+			file.delete();
+		}
 	}
 
 	@Test