This enabled HTTP time-based cache for storage assets, primarily
avatars. I have not observed If-Modified-Since from browsers during
tests but I guess it's good to support regardless.
It introduces a new generic httpcache module that can handle both
time-based and etag-based caching.
Additionally, manifest.json and robots.txt are now also cachable.
* Use mount but not register for chi routes
* Fix test
* Fix test
* Fix test
* Fix comment
* turn back unnecessary change
* Remove the timout middleware since some operations may spend much time.