Introducing UPDATE UPSERT

It would be nice to have a functionality similar to PostgreSQL or MongoDB's UPSERT.

UPSERT update a record if it already exists, or insert a new record if it does not - all in a single statement.
It should decreases time of data insertion, where the scenario 

if(SELECT record exists) {
  UPDATE record;
} else {
  INSERT record;
}

is common.
UPSERT operator has a sense with class name and WHERE operator. In other cases generates an exception.

Full UPDATE syntax would look following:
"UPDATE <class>|cluster:<cluster>> [SET|ADD|PUT|REMOVE|INCREMENT|CONTENT {<JSON>}|MERGE {<JSON>}] [[,] <field-name> = <expression>|<sub-command>]*  [LOCK <NONE|RECORD>] [UPSERT] [RETURN <COUNT|BEFORE|AFTER>] [WHERE <conditions>]