Flush append only buffers before existing.

when SHUTDOWN command is recived it is possible that some of the recent
command were not yet flushed from the AOF buffer, and the server
experiences data loss at shutdown.

src/server.c

@@ -2536,8 +2536,9 @@ int prepareForShutdown(int flags) {
                 "There is a child rewriting the AOF. Killing it!");
             kill(server.aof_child_pid,SIGUSR1);
         }
-        /* Append only file: fsync() the AOF and exit */
+        /* Append only file: flush buffers and fsync() the AOF at exit */
         serverLog(LL_NOTICE,"Calling fsync() on the AOF file.");
+        flushAppendOnlyFile(1);
         aof_fsync(server.aof_fd);
     }