Fix crash on serve exit, second attempt

[Wumpf]

Second attempt to #1613.

New commit is separate - passing shutdown bool around in order to not infect everything with tokio.
It seems we almost never need to hook it up, somewhat suspicious but I couldn't find any missing case.

Testing list. All of these were tested on 13aad68

• cargo r -p rerun -- ../nyud.rrd
• cargo run -p rerun --features web_viewer -- ../api_demo.rrd --web-viewer
• cargo run -p re_web_viewer_server
• cargo run -p api_demo -- --serve
• python ./examples/python/api_demo/main.py --serve
• python -m rerun --web-viewer ../objectron.rrd
• python ./examples/python/tracking_hf_opencv/main.py
• cargo run -p api_demo

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct

[emilk]

Nice solution!