cadc-inventory-db: artifact iterator auto commit hard to grok

[pdowler]

the set autocommit to false is very far removed from set autocommit to true, making it hard to analyse and prove the behaviour is correct. It violates the normal best practice of having the start trasnaction and either commit or rollback close together in the code.

Probably: refactor to merge the ArtifactIteratorQuery and the ArtifactResultSetExtractor into a single class that does both the query and the iteration. There is no good way to determine that the caller has abandoned an iterator - only code review can help - but that's the same as the best practice txn handling mentioned above.

If done right, this will provide a good example/template for other streaming query result implementations.