Fix CachedSessionFactory Race

Close the pool so that any sessions returned after the factory is
`destroy()`ed are closed.

* Call `removeAllIdleItems()` in `close()`.

* Close sessions in `SftpStreamingMessageSourceTests`.

**cherry-pick to all supported branches**

spring-integration-file/src/main/java/org/springframework/integration/file/remote/session/CachingSessionFactory.java

@@ -137,7 +137,7 @@ public class CachingSessionFactory<F> implements SessionFactory<F>, DisposableBe
 	 */
 	@Override
 	public void destroy() {
-		this.pool.removeAllIdleItems();
+		this.pool.close();
 	}
 
 	/**