re_datastore: IndexTable::latest_at early-exit on unknown component

[teh-cmc]

Due to the way we build out our scenes, a very common thing that happens is that we query an entity path specifically looking for components that it knows nothing about.
This is fine, as long as we make sure to have a fast early-exit on that path. We currently don't.

There are actually many many ways to do this, with varying trade offs.
Depending on how we do this, we might actually be able to re-use this datastructure in order to help with #453 too!

• IndexTables needs to keep track (efficiently) of what components they know of
• IndexTable::latest_at needs to early-exit appropriately

[teh-cmc]

Also, this is worth the introduction of an integration benchmark specifically targeting the use-case of querying for non-existing components.

[teh-cmc]

This actually should just work out-of-the-box with the new PoV query model.

Still need to add a benchmark for good measure tho.