[WIP] use other headers, when ETAG is missing

[dduenker]

Using last-modified seems pretty straightforward. I will take some time to figure out how to use expires and cache-control correctly.

The following material is intended to help me:

https://github.com/seanmonstar/reqwest/issues/368 https://github.com/zkat/make-fetch-happen https://github.com/gregjones/httpcache/blob/master/httpcache.go https://tools.ietf.org/html/rfc7234 https://github.com/kornelski/http-cache-semantics

[epwalsh]

Hey @dduenker, are you still planning on wrapping this up?