INT-4086: (S)FTP: Add Local File Tree Support

JIRA: https://jira.spring.io/browse/INT-4086

The remote files may be carried with sub-path, not just file name.
The same sub-dirs logic can be achieve with the `localFilenameGeneratorExpression`

* Add the logic into `AbstractInboundFileSynchronizer` to build sub-dirs for local files
* Switch on `WatchService` for the internal `FileReadingMessageSource` in the `AbstractInboundFileSynchronizingMessageSource` to react for all the changes in the `localDirectory`

Introduce `RecursiveDirectoryScanner` and use it for `AbstractInboundFileSynchronizingMessageSource` by default

* Add `maxDepth` and `fileVisitOptions` options to the `RecursiveDirectoryScanner`
* Polishing JavaDocs, tests and docs

spring-integration-file/src/test/java/org/springframework/integration/file/RecursiveDirectoryScannerTests.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 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.
+ * 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.file;
+
+import static org.hamcrest.Matchers.hasItem;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThat;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+import org.springframework.integration.file.filters.AcceptOnceFileListFilter;
+
+/**
+ * @author Iwein Fuld
+ * @author Gunnar Hillert
+ * @author Gary Russell
+ * @author Artem Bilan
+ *
+ * @since 5.0
+ */
+public class RecursiveDirectoryScannerTests {
+
+	private File subFolder;
+
+	private File subSubFolder;
+
+	private File topLevelFile;
+
+	private File subLevelFile;
+
+	private File subSubLevelFile;
+
+	@Rule
+	public TemporaryFolder recursivePath = new TemporaryFolder();
+
+	@Before
+	public void setup() throws IOException {
+		this.subFolder = this.recursivePath.newFolder("subFolder");
+		this.subSubFolder = new File(this.subFolder, "subSubFolder");
+		this.subSubFolder.mkdir();
+		this.topLevelFile = this.recursivePath.newFile("file1");
+		this.subLevelFile = new File(this.subFolder, "file2");
+		this.subLevelFile.createNewFile();
+		this.subSubLevelFile = new File(this.subSubFolder, "file3");
+		this.subSubLevelFile.createNewFile();
+	}
+
+	@Test
+	public void shouldReturnAllFilesIncludingDirs() throws IOException {
+		RecursiveDirectoryScanner scanner = new RecursiveDirectoryScanner();
+		scanner.setFilter(new AcceptOnceFileListFilter<>());
+		List<File> files = scanner.listFiles(this.recursivePath.getRoot());
+		assertEquals(5, files.size());
+		assertThat(files, hasItem(this.topLevelFile));
+		assertThat(files, hasItem(this.subLevelFile));
+		assertThat(files, hasItem(this.subSubLevelFile));
+		assertThat(files, hasItem(this.subFolder));
+		assertThat(files, hasItem(this.subSubFolder));
+		File file = new File(this.subSubFolder, "file4");
+		file.createNewFile();
+		files = scanner.listFiles(this.recursivePath.getRoot());
+		assertEquals(1, files.size());
+		assertThat(files, hasItem(file));
+	}
+
+}

spring-integration-file/src/test/java/org/springframework/integration/file/remote/synchronizer/AbstractRemoteFileSynchronizerTests.java

@@ -131,6 +131,7 @@ public class AbstractRemoteFileSynchronizerTests {
 		source.setMaxFetchSize(1);
 		source.setBeanName("maxFetchSizeSource");
 		source.afterPropertiesSet();
+		source.start();
 
 		source.receive();
 		assertEquals(1, count.get());
@@ -138,7 +139,7 @@ public class AbstractRemoteFileSynchronizerTests {
 		source.receive();
 		sync.synchronizeToLocalDirectory(mock(File.class), 1);
 		source.receive();
-		sync.close();
+		source.stop();
 	}
 
 	private AbstractInboundFileSynchronizer<String> createLimitingSynchronizer(final AtomicInteger count) {