- follow up to #2627
- use qa_num_browser_windows to set exact number of QA browsers,
fallback to qa_scale
- set num_browser_windows and num_browsers_per_pod using crawler / qa
values depending if QA crawl
- scale_from_browser_windows() accepts optional browsers_per_pod if
dealing with possible QA override
- store 'desiredScale' in CrawlStatus to avoid recomputing for later
scale resolving
- ensure status.scale is always the actual scale observed