Use multi statement when inserting documents

Currently, if the insert_behavior is as_nosql, that is, nosqlprotocol behaves just like MongoDB, there will be as many INSERT statements and roundtrips (between MaxScale and the backend) as there are documents.

If multi-statements are used, the number of roundtrips could be cut down to 1, which should have a significant impact on the performance.

If insert_behavior is as_mariadb, there will be just one INSERT statement and 1 roundtrip, which is likely to continue providing the best performance.