Add the $setOnInsert upsert operator

[alexeygeno]

Additionally:

• Emulating mongo behaviour for $set and $setOnInsert combined - https://stackoverflow.com/questions/23992723/findandmodify-fails-with-error-cannot-update-field1-and-field1-at-the-same/23994425#23994425

• Emulating mongo behaviour for $inc: it should increment even a non-existing field