stream feature locations

[michaelkirk]

This allows the client to start fetching FeatureData a little bit sooner.

TBH the measurable impact is pretty minimal in the browser, but I do see a little parallelism.

Before, on the left, you can see we don't fetch any Feature data until all the index traversal is complete.

On the right, you can see we start fetching Feature data a little earlier, before completing index traversal. Particularly interesting are those last two index traversal requests, which I think indicate dead end nodes - that is, nodes whose bounding boxes intersects ours, but who don't actually have any nodes in our bbox.