Ensure message keys have correct field order

See here: https://github.com/debezium/debezium/pull/1009#discussion_r328225165:

CREATE TABLE INFO (
    t1ID INT,
    t2ID INT,
    PRIMARY KEY (t2ID,t1ID)
) 

Then we'd end up with t1ID,t2ID instead of t2ID,t1ID as the Kafka message key.