mysql: use numeric comparison for binlog filename

[lance6716]

support upstream has more than 999999 binlog files

MySQL [(none)]> show binary logs;
+-------------------+-----------+
| Log_name          | File_size |
+-------------------+-----------+
| mysql-bin.999998  |       177 |
| mysql-bin.999999  |       779 |
| mysql-bin.1000000 |       444 |
+-------------------+-----------+
3 rows in set (0.003 sec)

[lichunzhu]

LGTM