Add TTL for cache items.

[btimby]

Currently items in the filesystem cache will live forever. There is no size limit nor is there a time limit.

Add a TTL (probably around 30s) so that cache objects will drop out after a short while. The idea of the cache is to reduce API calls, but not to eliminate them, also not at the expense of causing problems.

The reason I left this out is that I want the caching to be smart enough to avoid these problems altogether. I am hesitant to implement this, but if I see caching issues that cannot be individually addressed this may be the best solution.