Clear cache on zoom change

[henryptung]

This is somewhat more questionable and not necessarily something you want to merge into master, but I noticed that with significant panning and zooming over time, the UtfGrid layer developed a very large cache of all tiles ever fetched. This adds a zoomend handler that, on zoom change, deletes cache entries that don't match the current zoom, to minimize memory usage at runtime.

As a tradeoff, this means that zooming to level A, B, then back to A will cause tiles to reload from backend whereas they previously did not. Up to you on whether you think it's important.

[danzel]

Hrm ok thanks.
Maybe we want to have a LRU cache instead.

[henryptung]

So, I realized that I hadn't cleaned up my shapefile before loading, so I was getting a LOT of large metadata cruft on the frontend in a lot of tiles. The burden got significantly lighter once I stopped doing that, so I don't think this is nearly as important. Up to you on whether you want to merge, keep around a branch, or close out (it's still listed under pull requests, anyway).