Update max.iteration.transactions to a sensible default

For the SQL Server connector, setting the default value of `max.iteration.transactions` to `0` doesn't appear to be ideal. Multiple users have reported encountering OOM errors due to this configuration.

To recover the connector, it is often advised to set the configuration to a positive value so that a limited number of changes are fetched in each iteration and the memory usage can be bounded.

Proposing to set the default to 500 to avoid such issues.