Changes

Summary

1. merged
2. proper close;
3. separate host.db for independent db.transaction_lock;
4. tuned comments;
5. omit redundant data: already stored in progress database;
6. tuned signature: more operations;
7. proper support for SQLite: avoid conflicts on transaction_lock;
8. minor performance tuning: avoid external process;
9. clarified signature;
10. obsolete;
11. tuned;
12. tuned signature;
13. tuned signature;
14. clarified signature;
15. tuned;
16. clarified signature;
17. tuned;
18. proper pattern (amending da5cc332ded3);
19. tuned signature;
20. proper hostname from build_context;
21. prefer Database_Progress, which is more robust (amending afb1a19307c4);
22. support for Database_Progress;
23. tuned signature: more operations;
24. tuned signature: more operations;
25. tuned signature: more operations;
26. clarified signature;