Split apart the thin inheritance relationship of MergeDetectionsTask and MergeMeasurementsTask by removing MergeSourcesTask. The separated classes should be pulled out into their own files in the refactoring.

This change both makes each class more manageable on its own, but will also simplify the conversion to PipelineTasks.

This ticket will not remove the MergeSourcesTaskRunner, as it will only be around while the gen2 middleware exists anyway.