* Implement a new package: engine. It exposes a useful but minimalist job API.
* Refactor main() to instanciate an Engine instead of a Server directly.
* Refactor server.go to register an engine job.
This is the smallest possible refactor which can include the new Engine design
into master. More gradual refactoring will follow.
Upstream-commit: 0d1a825137448e2f41e5aaa5ecae8094c8ab6817
Component: engine