Osu: Quick retry should be instant, it shouldn't reload the entire beatmap

[LittleEndu]

Describe the bug:
When restarting the map (via quick retry hotkey) everything is reset and reloaded which can take a very long time on low spec computers or on maps with elaborate storyboards. Instead of restarting the entire map, only score (and replay recording, etc.) should reset and song should be seeked to the beginning, everything else that stays the same between runs (hitobjects, storyboard assets) shouldn't be reloaded.

osu!lazer version:
2020.515.0

[peppy]

There's two ways to look at this. First of all we do have an eventual plan of caching the post-parsed beatmap content in WorkingBeatmap. This would help improve load time even after returning to song select (until the working beatmap was GC'd).

Second is that a quick restart only needs to rewind to the beginning of the beatmap, and we may be able to do this in a fun way using existing rewind logic. Unfortunately rewind logic is also under-performant current and this would likely take longer (unless near the beginning of the map) currently.

Both relatively low priority but definite considerations we have in mind that will help with the mentioned restart case.