refactor to have separable build servers

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.

The issue provides an architecture sketch but names no files, tests, or entry points. Begin by mapping the current build and cache flow, then compare it with the proposed CDN, app-level lookup, Gearman server, Redis cache, and separated builder workers. Done would require an agreed implementation scope and a working architecture that supports independent builders and cache busting.

Written by the indexing model from the issue text.