Regression in master: SET transaction_isolation = 'repeatable_read';

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In this PR https://github.com/github/gh-ost/commit/da3514253f5cd848c1cbcaad63ad794427ed1833

`GetDBUri()` will execute a statement such as the following:

```
set transaction_isolation = 'repeatable-read';
```

This variable was only [introduced in MySQL 5.7.20](https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_transaction_isolation), so if you are using an earlier version such as what Aurora 5.7 is, it will result in an error:

```
mysql> set transaction_isolation = 'repeatable-read';
ERROR 1193 (HY000): Unknown system variable 'transaction_isolation'
```

Instead the statement needs to be `set tx_isolation = 'repeatable-read'`, but this statement is not supported by MySQL 8.0. So some version-specific logic is probably required.