Include event scn in Oracle records

In the records generated by the Oracle source connector it would be useful to have the LogMiner event scn included in the `source` for after-the-fact processing of the change events.

Using 1.9.3.Final there is an scn field which provides the low-watermark of the oldest in progress transaction. This means that all change events produced during this period have the same scn value as the low-watermark isn't changing.

The suggestion from the chat was to potentially include a new event_scn field in the source info block.