Parallel WeakProcessor may use too few threads

WeakProcessor::ergo_workers is used to determine the number of threads to use for parallel weak processing. It should be called with the maximum number of threads to use, and may scale that down based on the number of weak references and ReferencesPerThread.

However, the value it is being called with is the active_workers() value of the work gang, rather than the total_workers(). Because of this, the result may be more limited than intended, leading to longer processing times.