Robust file cache

[annehaley]

I think this merits adding a cache abstraction, i.e. functions to mediate adding to the cache that internally handle eviction, rather than direct access to the underlying data structure.

Originally posted by @zachmullen in #597 (comment)

See above PR for details on what further action is needed.

[zachmullen]

The most important aspect of this is making sure we do cache eviction of previously viewed images. We should only keep the m most recently viewed frames, and only prefetch the next n frames in the default order. Cache entries will need to also contain a flag indicating whether they are fully loaded, or pending.