Refactored SimpleSketchGenerator to use SketchFlow.

SketchFlow defines the flow through the update graph, issuing retrieval queries, collecting count statistics, and updating sketch distributions

SketchTrace is a tracing interface through the SketchFlow. Access to any of the variables, or alternative dump code for interfacing with Galago Eval goes here.

SketchFlow is still missing a few pieces, e.g. entity counting. Also, relations and categories are not even started yet.