AliSQLBackup depends on tokudb_checkpoint_lock to implement hot backup, but

invoking ft_close() to a FT file during backup will break the consistency

between this FT and the others in the final backup directory, even when

checkpoint lock is held, becasue the header of FT is updated by ft_close().

This patch checks whether we are in backup before closing FT, if in backup,

don't close FT.