Zooming in on the poles causes non-stop imagery requets

Reported on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/5C2Yx_5KUMQ

Zoom into the poles and watch the network requests, especially compared to the same zoom level in other areas of the world, and can make applications around the arctic areas perform poorly.

Start by reproducing the issue described in the linked forum report: zoom into the poles and monitor network requests, comparing them with the same zoom level elsewhere. Trace the imagery request behavior from the relevant JavaScript entry point; done means polar zooming no longer produces non-stop requests or causes poor application performance.

Written by the indexing model from the issue text.