Improve client-side caching of frames

[hackermd]

Find an optimal balance between caching frames (tiles) client-side and memory footprint. We may want to configure this differently depending on the size of a "slide" (i.e., the number of associated instances or frames) or handle it differently for different pyramid image resolution levels (e.g., cache top pyramid layer).

This may be critical if certain pyramid levels are missing server side. For example, some of the Aperio SVS files only contain 4x, 20x, and 40x images and lack a single-frame 1x or 2x image.

[fedorov]

After discussing this, we believe we do not have the specific examples in IDC or otherwise that would justify efforts to investigate this issue.