Improve discrete event performance during gridlock

Follow https://dabreegster.github.io/abstreet/trafficsim/gridlock.html#strategy-for-resolving until the full day can complete at all.

Currently:

```
cargo run --release --bin headless -- --disable_turn_conflicts --infinite_parking --disable_block_the_box --port=1234 --alerts=silence

curl http://localhost:1234/sim/load -d '{ "scenario": "data/system/scenarios/downtown/weekday.bin", "modifiers": [{"ChangeMode":{"to_mode":"Drive","pct_ppl":100,"departure_filter":[0.0,86400.0],"from_modes":["Bike"]}}], "edits": null }' -X POST

curl http://localhost:1234/sim/goto-time?t=08:00:00
```

Things keep flowing (!!) but this is so incredibly slow. Time for profiling.