SIMPLY-2627 mentions a number of different types of resources, most of which can and should have an associated value for the Cache-Control header. We need to define appropriate caching times for these resources. These will be codified in source code but it'll be good to have them written down in the issue tracker for later reference.

"OPDS feeds describing lists of books" includes a lot of different types of feeds – recommendation feeds, search results, etc. Each of these currently has its own OPDSFeed and/of Facets subclass, and many of those classes already define a MAX_CACHE_AGE configuring how long a generated feed should be cached on the server side. We could address all of these at once by deciding that the Cache-Control header (which controls caching on the client side) is set to some fraction of the MAX_CACHE_AGE.