Switch to real source for DomainScore tuples

[kkrugler]

This needs to be a source that never emits anything, but stays "active" until terminated so our iteration of DomainScore tuples works. Currently we're using the collection-based source, but this should be for testing only, and has a fixed parallelism of 1, which means our windowing code for calculating average domain page score is also forced to use a parallelism of 1.