Result of performance spike T124331

As a low-hanging fruit, we decided to set a naive limit on the count of images stored in memory. Although NSCache doesn't guarantee when (if ever) it will purge objects from memory when thresholds are exceeded, this should (at least in theory) prevent image usage in the app from causing memory warnings as often as they currently do.

Acceptance criteria

• Set a limit on WMFImageController's internal memory cache (SDImageCache.maxMemoryCountLimit)
• Test the app to see if count limits are respected by the cache
• Verify that operating the app with the cache near its count limit doesn't cause excessive "thrashing" of the cache repeatedly purging and re-storing the same images (i.e. should purge in LRU order)