refactor to have separable build servers

[jfhbrook]

I think the end architecture should look something like,

--->(Actual Real CDN)--->(app-level cache/lookup)--->(gearman server)--->(builder workers)

The CDN can only cache specific routes. The app-level cache/lookup is still required for a number of reasons. Ideally, these would use a redis and there would be an extra worker for cache busting. Using gearman to separate off the builders nets an added bonus of being able to use docker or similar to sandbox.

[jfhbrook]

One idea: If/when gearman's server API is implemented in node, we can make the cache/lookup and the server one piece.