Changes

Summary

1. tuned whitespace;
2. tuned;
3. clarified signature, although "sql" argument is de-facto mandatory;
4. tuned;
5. proper SQL (amending 7ab9bac1ca96);
6. clarified signature: more explicit "synchronized" regions;
7. more robust interrupt handling, notably for Build_Job.terminate();
8. clarified signature: works for general Build_Job;
9. tuned;
10. clarified modules;
11. clarified signature;
12. proper log_lines, without protocol messages (amending cb3f5361fbca);
13. clarified signature;
14. tuned messages;
15. clarified error output vs. process_result stored in build_database (see also 13a0f537e232 and bff56eae3ec5);
16. clarified system option: guard for testing, until the database layout has stabilized;