Implement $setOnInsert update operator

[tatu-at-datastax]

One more update operator in-scope for first version is $setOnInsert: similar to $set but only applying on actual inserts, not updates.

Expected behavior should be similar to what is shown here:

https://www.mongodb.com/docs/manual/reference/operator/update/setOnInsert/#mongodb-update-up.-setOnInsert

[tatu-at-datastax]

Note: detection of whether a document to update or insert can be found from ReadAndUpdateOperation;

                  DocumentUpdater.DocumentUpdaterResponse documentUpdaterResponse =
                      documentUpdater().applyUpdates(readDocument.document().deepCopy());
                  final JsonNode originalDocument =
                      readDocument.txnId() == null ? null : readDocument.document();

where missing txnId() indicates a newly inserted document