"source" block for Postgres events shouldn't contain "last_snapshot_record" field

This is only needed within the KC offset to detect an on-going snapshot after a restart, but it's not needed within "source" in the actual message payload. Ideally, when changing this, the PG connector should make use of the same structure as the Oracle one, which clearly separates between SourceInfo (representing the "source" element of messages) and OffsetContext (managing the KC offset and links to SourceInfo).