INT-2898 Add Persistent FileListFilters

- Abstract implementation
- Implementation for the file system
- Implementation for remote files ((S)FTP)

JIRA: https://jira.springsource.org/browse/INT-2898

INT-2889 Polishing

- Add (S)FTP test cases
- Docs

src/reference/docbook/file.xml

@@ -35,6 +35,14 @@
       <interfacename>FileListFilter</interfacename>. By default, an
       <classname>AcceptOnceFileListFilter</classname> is used. This filter
       ensures files are picked up only once from the directory.
+      <note>
+        The <classname>AcceptOnceFileListFilter</classname> stores its state in memory. If you wish the
+        state to survive a system restart, consider using the
+        <classname>FileSystemPersistentAcceptOnceFileListFilter</classname> instead. This filter stores
+        the accepted file names in a <interfacename>MetadataStore</interfacename>. The framework supplies
+        several store implementations (such as Redis), or you can provide your own. This filter matches on
+        the filename and modified time.
+      </note>
       <programlisting language="xml"><![CDATA[<bean id="pollableFileSource"
     class="org.springframework.integration.file.FileReadingMessageSource"
     p:inputDirectory="${input.directory}"