Sink: Missed updates when using subdoc mode with append/prepend

A connector configured to use SUBDOCUMENT mode with an operation of `ARRAY_PREPEND` of `ARRAY_PREPEND` could mistakenly ignore updates if Kafka records targeting the same document arrive in rapid succession.

This happened because the connector was using an event deduplication algorithm based on the document ID: if two records targeting the same document arrive in the same batch, the first record was dropped. The connector fails to prepend/append the items from the dropped record.