Fetch responses along tile boundaries

We already use tile boundaries at zoom level 17 to request parcel data. Now we use the same segmentation to request responses. Technically, we'll usually request data for a slightly larger area than before. But in practice, most of that data will not have changed, so we'll receive a very small 304 response from the server. That's especially helpful with the small pans that are common with usage of the mobile app.

/cc @hampelm