Backfill the cache on a miss

[carolynvs]

I am also stealing the glory and including Marwan's /latest endpoint.

[arschles]

Ref #290

[codecov-io]

Codecov Report

Merging #342 into master will decrease coverage by 0.06%.
The diff coverage is 70%.

@@            Coverage Diff            @@
##           master    #342      +/-   ##
=========================================
- Coverage   37.47%   37.4%   -0.07%     
=========================================
  Files         104     104              
  Lines        2858    2863       +5     
=========================================
  Hits         1071    1071              
- Misses       1679    1684       +5     
  Partials      108     108

Impacted Files	Coverage Δ
pkg/download/goget/goget.go	29.03% <0%> (ø)	⬆️
cmd/proxy/actions/app_proxy.go	100% <100%> (ø)	⬆️
cmd/olympus/actions/app.go	80.95% <100%> (ø)	⬆️
pkg/module/go_get_fetcher.go	49.47% <57.14%> (+0.01%)	⬆️
cmd/proxy/actions/app.go	65.26% <66.66%> (+0.74%)	⬆️
pkg/repo/github/commitinfo.go	64% <0%> (-12%)	⬇️
cmd/proxy/actions/cache_miss_handler.go	0% <0%> (-3.71%)	⬇️

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