INT-3737: (S)FTP Outbound Gateway Streaming GET

JIRA: https://jira.spring.io/browse/INT-3737

Support returning an `InputStream` from a GET operation.

This can be used in conjuction with a `<file:splitter/>` to stream a text file.

The user is responsible for releasing the session when the download is complete.

The session object is stored in the message header and `RemoteFileUtils.closeSession()` should be called
to clean up and close the session.

INT-3737: Polishing and Docs

Fix `CachedSession.close()` bug

Preventing `Caused by: java.io.IOException: Previous raw read was not finalized`
when `Session` is returned to the pool, but `readingRaw` hasn't been finalized
because the real `close()` isn't invoked in case of normal return to pool.

spring-integration-ftp/src/main/java/org/springframework/integration/ftp/session/FtpSession.java

@@ -144,6 +144,9 @@ public class FtpSession implements Session<FTPFile> {
 	@Override
 	public void close() {
 		try {
+			if (this.readingRaw.get()) {
+				finalizeRaw();
+			}
 			this.client.disconnect();
 		}
 		catch (Exception e) {