Mitigate renderer memory usage

attempts: first try in https://gitlab.eox.at/esa/prism/vhr_image_2018/-/commit/4f6420dde5658c9d54fbafd766cc1f8a99747b79 - later increased to 8GB

then also currently -max-requests 10 --max-requests-jitter 3 added to gunicorn in 25ff31fc

Further investigation can be done.