Changes

Summary

1. clarified signature: reduce boilerplate;
2. clarified messages;
3. tuned signature;
4. tuned structure;
5. clarified signature;
6. clarified signature;
7. clarified build process roles: "worker" vs. "build";
8. clarified database content; tuned signature;
9. tuned: prefer iterator.nextOption;
10. tuned whitespace and braces;
11. clarified signature: more uniform operations;
12. tuned signature: reduce boilerplate;
13. tuned signature;
14. proper clean_build of old data at start of new process --- allow to inspect remains of the last process;
15. more database content: formal end_build;
16. more operations;
17. clarified database content and prepare/init stages;
18. tuned signature;
19. tuned;
20. tuned;
21. less verbosity, amending 3bc49507bae5;
22. tuned comments; tuned structure;
23. tuned signature: avoid totally adhoc overriding;
24. separate static build_uuid from dynamic worker_uuid, to allow multiple worker processes participate in one build process;