In a Dart query, all Historicals are given worker IDs, but not all of them
are going to actually be started or receive work orders. This can create gaps
in the set of workers. For example, workers 1 and 3 could have work assigned
while workers 0 and 2 do not.
This patch updates ControllerStageTracker and WorkerInputs to handle such
gaps, by using the set of actual worker numbers, rather than 0..workerCount,
in various places.
06bbdb38ce