### What
* Closes #2517

The datastore is not the only place we store data. For each node in the
entity tree we store a _time histogram_ of where we have data. This was
never properly purged post GC - a very rough heuristic was instead used
(throw away everything up to the oldest time in the store - which will
be an even worse heursistic after
#3357).

With this PR, the store gc will book-keep exactly what was deleted, and
it will be properly purged from all secondary indices.

The resulting code is a bit complicated, because the store has no idea
about the hierarchical nature of the entity paths, but we store the time
histograms (and other book-keeping) for each node. That is, logging to
`/foo/bar` we will note the data on `/`, `/foo` and `/foo/bar`, but the
data will only be registered in the store for `/foo/bar`.

I also fixed a bunch of other smaller things that came up.

Best reviewed commit-by-commit.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/3364) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/3364)
- [Docs
preview](https://rerun.io/preview/18cbc53fe96798cbe45ab84ddeb66aa183253e12/docs)
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- [Examples
preview](https://rerun.io/preview/18cbc53fe96798cbe45ab84ddeb66aa183253e12/examples)
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- [Recent benchmark results](https://ref.rerun.io/dev/bench/)
- [Wasm size tracking](https://ref.rerun.io/dev/sizes/)