MXS-2495 Cleanup GWBUF interface

All GWBUF macros that address a single link in a chain are now
simple wrappers for equivalent gwbuf_link-functions.

Next step is to drop the macros and replace their use with calls
to the functions.

server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc

@@ -729,7 +729,7 @@ static inline bool collecting_resultset(MySQLProtocol* proto, uint64_t capabilit
  */
 static inline bool not_ok_packet(const GWBUF* buffer)
 {
-    uint8_t* data = GWBUF_DATA(buffer);
+    const uint8_t* data = GWBUF_DATA(buffer);
 
     return data[4] != MYSQL_REPLY_OK
            ||   // Should be more than 7 bytes of payload