merged

revert most parts of 0e79fa88cab6: somewhat ambitious attempt to move towards "editing" builds via added/canceled workers; retain notion of next_jobs.limit and finished() from 0e79fa88cab6; clarified Job vs. optional Build_Job;

database performance tuning: prefer light-weight IPC over heavy-duty transactions, following bf377e10ff3b;

tuned;

tuned whitespace;

proper system option, instead of hardwired default;