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
a2d1edfbb9