Schema changes should flush SCN to offsets if there are no other active transactions

There is a corner case where a connector can be started and the only event that it receives is a DDL event before being stopped. If the connector is then restarted, the SCN read from the offsets will not be that of the DDL event but instead will be that from the snapshot.

We should always flush the SCN to the offsets under the following two conditions:

1. No active transaction
2. Only one active transaction, where the transaction is the one that wraps the DDL