MDEV-9058: protocol: COM_MULTI command (part 1)

Adding a command from the end of avaliable commands numering space (254)

include/mysql.h.pp

@@ -9,6 +9,9 @@ enum enum_server_command
   COM_TABLE_DUMP, COM_CONNECT_OUT, COM_REGISTER_SLAVE,
   COM_STMT_PREPARE, COM_STMT_EXECUTE, COM_STMT_SEND_LONG_DATA, COM_STMT_CLOSE,
   COM_STMT_RESET, COM_SET_OPTION, COM_STMT_FETCH, COM_DAEMON,
+  COM_MDB_GAP_BEG,
+  COM_MDB_GAP_END=253,
+  COM_MULTI,
   COM_END
 };
 struct st_vio;