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
96ca9647a4