Terrain and imagery fixes

Most importantly, this fixes a regression introduced in #1737 which broke the time-slicing logic for terrain and imagery loading.  All tiles in the load queue were always processed every frame, leading to framerate choppiness when moving the camera around.

I also changed the implementation of `throttleRequestByServer` to use the `Uri` class to extract the server for a URL, instead of using an anchor tag which is actually slower.

Plus some other miscellaneous cleanup.