Use -race build flag

Lean on the tooling.
Add groupcache dependency

singleflight looks like what I want to use.
Introduce worker pool for backend API

I don’t think this is thread-safe, but it’s a start towards
deduplicating backing service requests.

Approach inspired by http://blog.gitorious.org/2014/08/11/golang-patterns-for-serving-on-demand-generated-content/
Merge pull request #4 from alphagov/feature/bounded-workers

Introduce worker pool for backend API