add wal interface

src/vnode/wal/inc/vnodewal.h

-#if !defined(_TD_WAL_H_)
+/*
+ * Copyright (c) 2019 TAOS Data, Inc. <jhtao@taosdata.com>
+ *
+ * This program is free software: you can use, redistribute, and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3
+ * or later ("AGPL"), as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef _TD_WAL_H_
 #define _TD_WAL_H_
+#include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
+typedef void walh;  // WAL HANDLE
 
-#endif // _TD_WAL_H_
+walh *vnodeOpenWal(int vnode, uint8_t op);
+int   vnodeCloseWal(walh *pWal);
+int   vnodeRenewWal(walh *pWal);
+int   vnodeWriteWal(walh *pWal, void *cont, int contLen);
+int   vnodeSyncWal(walh *pWal);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // _TD_WAL_H_