evaluate replacing current caching library

[jondot]

Suggestion / Feature Request

The current caching library works well and was very easy to use: https://github.com/rusty-ferris-club/backpack/blob/master/backpack/src/fetch.rs#L48

It is used in a forced way where the cache is kept static for a fixed amount of time. However a smarter strategy would be to:

1. Have a short-lived cache with no network request (e.g. 1 day)
2. When (1) expires, perform a cheap Etag/HEAD request to see if anything changed, and expire the cache if needed
3. After a rather long period of time (e.g. 14 days) expire the cache in any case

Note:

We fetch content through git and through HTTP.

• HTTP has natural caching facilities that can be used
• see what's the best way to cache around git

This issue is for evaluation, so a good completion would be to just summarize your findings. A great completion of it would be a working implementation of a new cache mechanism.