Study how to improve performance of metadata evaluation

Some background (Windows):

• 13000+ control scripts to be evaluated (passed to QJSEngine)
  • if selecting all repository categories in the online installer, depends also on your license
  • this number is growing with each Qt/B2Qt/etc. release

• Each meta.7z containing the control (.qs) script is downloaded first (using multiple threads)
• Each meta.7z is then extracted, in parallel
• Evaluation of the scripts is done one by one
  • scripts (the installer components they belong to) may have dependencies
  • this takes roughly 3 minutes

Currently, the worst-case scenario is on Windows when selecting all categories. It takes about ~8 mins to download, extract and evaluate all the 13000+ scripts, of which ~3 mins is for the evaluation (QJSEngine).

Suggestions on how to improve the situation:

• Run the whole thing with profiler to check where time is actually spent
• keeping and re-using the *.jsc cache files the JS engine creates in the process
• minimize wrapping and unwrapping QObjects into and out of QJSValue
• run multiple QJSEngine instances in parallel