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-sftp/src/test/java/org/springframework/integration/sftp/inbound/SftpInboundRemoteFileSystemSynchronizerTests.java

@@ -121,6 +121,8 @@ public class SftpInboundRemoteFileSystemSynchronizerTests {
 		localFileListFilter.addFilter(localAcceptOnceFilter);
 		ms.setLocalFilter(localFileListFilter);
 		ms.afterPropertiesSet();
+		ms.start();
+
 		Message<File> atestFile =  ms.receive();
 		assertNotNull(atestFile);
 		assertEquals("a.test", atestFile.getPayload().getName());