Add caching of original images and responses

[aaron42net]

Potentially use https://github.com/gregjones/httpcache which has flexible storage options.

[aaron42net]

I implemented httpcache, to the point of writing an LRU in-memory cache implementation (https://github.com/die-net/lrucache) for it. Unfortunately, httpcache expects to be a client cache which causes a number of minor mismatches with its use in a proxy.

I may investigate https://github.com/lox/httpcache next, which is designed as a server-side cache.