dms_conversion_catalog: use proper activity queue
Because this is using SQLDict, when we had multiple conversion activities for the same path with different conversion parameters, only one was executed.
We don't want this here, as the recent failures on testDmsWithPreConversion.TestDocumentWithPreConversion are showing, when the same image is referenced with different conversion parameters we want to pre-convert for all conversion parameters.
Use SQLQueue, which allows calling the same method more than once.