GH-3488: Fix Persistent Filters with Recursion

Resolves https://github.com/spring-projects/spring-integration/issues/3488

Resolves two problems:

- When changes are made deep in the directory tree, they were not detected because
  the directory is in the metadata store and only passes the filter if a file
  immediately under it is changed, changing the directory's timestamp.

This is solved by subclassing `AbstractDirectoryAwareFileListFilter`, allowing its
`alwaysAcceptDirectories` property to be set.

- Only the filename was used as a metadata key; causing problems if a file with the
  same name appears multiple times in the tree.

This is solved with a new property on `AbstractDirectoryAwareFileListFilter` used by
the gateways to determine whether to filter the raw file names returned by the session
(previous behavior) or the full path relative to the root directory.

**cherry-pick to 5.4.x, 5.3.x**

* Some code style clean up

# Conflicts:
#	spring-integration-file/src/main/java/org/springframework/integration/file/remote/gateway/AbstractRemoteFileOutboundGateway.java
#	src/reference/asciidoc/whats-new.adoc

spring-integration-ftp/src/main/java/org/springframework/integration/ftp/filters/FtpPersistentAcceptOnceFileListFilter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2019 the original author or authors.
+ * Copyright 2013-2021 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.
@@ -27,6 +27,7 @@ import org.springframework.integration.metadata.ConcurrentMetadataStore;
  * 'seen' this file.
  *
  * @author Gary Russell
+ *
  * @since 3.0
  *
  */
@@ -46,4 +47,9 @@ public class FtpPersistentAcceptOnceFileListFilter extends AbstractPersistentAcc
 		return file.getName();
 	}
 
+	@Override
+	protected boolean isDirectory(FTPFile file) {
+		return file.isDirectory();
+	}
+
 }