add flush to gateway

modules/elasticsearch/src/main/java/org/elasticsearch/index/translog/Translog.java

@@ -87,6 +87,11 @@ public interface Translog extends IndexShardComponent {
      */
     Snapshot snapshot(Snapshot snapshot);
 
+    /**
+     * Flushes the translog.
+     */
+    void flush();
+
     /**
      * Closes the transaction log.
      */

modules/elasticsearch/src/main/java/org/elasticsearch/index/translog/fs/FsTranslog.java

@@ -185,6 +185,18 @@ public class FsTranslog extends AbstractIndexShardComponent implements Translog
         }
     }
 
+    @Override public void flush() {
+        synchronized (mutex) {
+            if (raf != null) {
+                try {
+                    raf.raf().getFD().sync();
+                } catch (Exception e) {
+                    // ignore
+                }
+            }
+        }
+    }
+
     @Override public void close(boolean delete) {
         synchronized (mutex) {
             if (raf != null) {