Align field names in source info block across connectors

In the source metadata in CDC messages we have multiple fields which are common to all connectors but have slightly different names, e.g. "sec" vs. "ts_sec" or "snapshot" vs. "initsync".

This ought to be stream-lined before going to 1.0. Of course specific connectors will still be able to add their custom fields, but the common ones should be represented uniformely.