Avoid aggregate_assets_summary upon every version save

[yarikoptic]

triggered by @mvandenburgh have written in #1372

The main contributor to the potentially long validation time is the fact that we loop over every Asset in a Version when it's being saved https://github.com/dandi/dandi-archive/blob/master/dandiapi/api/models/version.py#L237-L242.

which calls out to aggregate_assets_summary.

If that is the case, and we do save version to update its modified upon each asset validation since https://github.com/dandi/dandi-archive/pull/1366/files -- I feel like we might be piling up lots of actions we should avoid or "debounce". What does happen when I upload e.g. 10 assets -- will we have 10+ aggregate_assets_summary guaranteed to be ran?

[mvandenburgh]

Closed by #1431