This allows AppsReadFS/AppsReadWeb to be used behind the scenes of this
API for the conditional loading logic. All functions are left as public
for now while we're experimenting.
This logic supports the following cases:
- Download a fresh apps.json and load it if missing
- Check if a local apps.json is old and get a fresh one if so
- Always save a local copy after downloading a fresh apps.json
The http.Head() call is faster than a http.Get() call (only carries back
respones headers) and aims to make the more general case more
performant: you have the latest copy of the apps.json and don't need to
download another one. This a direct port of our Bash implementation
logic.
Closes https://git.autonomic.zone/coop-cloud/go-abra/issues/9.
