Automatically enable sync_transaction if throttling starts

MXS-6572 adds the foundation for transaction throttling by adding a server status bit that when detected by readwritesplit will cause it to throttle the transactions if it's configured in the appropriate configuration.

The only problem is that the configuration is not the default configuration and it would be beneficial for the throttling to be possible even without manually enabled transaction synchronization.

The goal of this task is to make it so that in the default readwritesplit configuration, which has sync_transaction=none, the throttling will start with a pre-defined configuration if it's not yet enabled.