Update to qgis_bench: use renderer job, allow parallel mode, print other time
Fix unit tests

- QgsFeature owns QgsFields (otherwise the assigned QgsFields may get deleted)
- updated QgsRenderChecker to use map settings + map renderer job
- fixed DPI issue in map renderer to map settings conversion
- fixed vector layer cache (failing file operations)

There are still few tests that keep failing compared to master branch,
but the reason for failure are tiny pixel diffs and we should probably just
update the expected images.
User-configurable update interval
Optionally draw little rendering stats (switch on/off with S key)