add GCS cache

[diegomarangoni]

Add new option to cache assets on Google Cloud Storage

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[diegomarangoni]

I signed it!

[googlebot]

CLAs look good, thanks!

[diegomarangoni]

@willnorris do you know why my build is not passing?

[diegomarangoni]

looks like some libs are not working with Go 1.5 anymore.

[willnorris]

looks like some libs are not working with Go 1.5 anymore.

hmm, I suspect I broke that when I upgraded the vendored packages. :-\ As long as we're compatible with 1.8, I think we're fine though.

[diegomarangoni]

Hope it is, I am running in production lol
Serving 240GB with no problems

BTW, I didn't know how to correctly set vendor.json, so I leave without it.

[willnorris]

uh, wow... vendoring all the transitive dependencies brings in 57 new packages weighing about 4 MB. More concerning is that the resulting imageproxy binary doubles in size from ~7 MB to ~14 MB. Not your fault at all! this is just how the google cloud sdk is written. I'm just not sure how I feel about that :-\

[diegomarangoni]

maybe writing as a plugin?
well, 14MB still considerable small for me...
may turn into a mess if the number of functionalities increases.

[willnorris]

shoving the entire JSON key into an environment variable seems a little weird, but I guess it works, and is at least consistent with the other caching backends :) We can always adjust that later if needed. I'll rework these commits slightly and merge shortly.

[willnorris]

well, oops. I tried updating your branch with the updated commits and ended up breaking something which automatically closed this PR. That was weird. In any event, everything is now merged in.

[diegomarangoni]

thanks @willnorris
upgrading my image to latest version ;)