re_datastore: optimize backwards walk in read path

[teh-cmc]

This currently does a naive linear walk for as long as the component isn't set:

            .filter_map(|(name, index)| {
                let mut row_idx = time_row_idx;
                while !index.is_valid(row_idx as _) {
                    row_idx -= 1;
                    if row_idx < 0 {
                        return None;
                    }
                }

                assert!(index.is_valid(row_idx as usize));
                (*name, index.values()[row_idx as usize]).into()
            })

Should be able to make that O(log(n))-ish using a B-tree or something.

[teh-cmc]

And now it does it at the bucket level too! :)

[teh-cmc]

Irrelevant now.