The worker server was heavily tied to OSBuildJob(Result). Untie it so
that it can deal with different job types in the future.
This necessitates a change in the jobqueue: Dequeue() now returns the
job type, as well as job arguments as json.RawMessage. This is so that
the server can wait on multiple job types with different argument
types.
The weldr, composer, and koji APIs continue to use only "osbuild" jobs.