cancellation of tile loads

[incanus]

On very fast zooming on both platforms, tile loads still have to complete fetch before successive ones can run through. We should expire and cancel actual network fetches, not wait until the fetch is complete regardless then possibly throw the tile away before render.

[incanus]

One simple approach here would be to assume that incoming requests for tiles at a given zoom will obsolete in-progress downloads of tiles from other zooms. Then the renderer itself isn't responsible for posting cancellations and platform::request_http could just manage this entirely internally.

Pan is likely not as much of an issue as both mouse & touch gestures are related to screen and trackpad size, whereas zooms are related to potentially very fast trackpad/wheel, quickstart pan, and/or tap gestures.

[incanus]

Based on the likely outcome of OS versions in #75 I am going to switch over to NSURLSession to make this easier.

[incanus]

Pull happening in #76.