Change execution order for single-threaded server

[pavpanchekha]

Right now, the single-threaded server runs jobs when start-job is called. This PR changes it to instead run jobs when wait-for-job is called. The reason to change things is simple; run.rkt has code that at a high level does:

(for ([job ...])
  (start-job job))
(for ([job ...])
  (wait-for-job job)
  (print-status job))

If we run things in start-job then we get all the status prints at the very end, once all jobs are done (useless). If we run then in wait-for-job then we get prints one at a time, like we're suppose to.

The PR also has some random minor improvements to the server code.