We were terminating after the first set of jobs; if one of them scheduled
another job, and there were no timers running, we would terminate,
which was wrong.
If a job enqueues another job on the executor, we might never leave
the inner `while` loop in the `run()` method. Fix this by taking
the contents of the run queue and only running those jobs in the
queue at the time we enter the inner loop.
