Changes

Summary

1. disable write_updates from f425bbc4b2eb for now: "isabelle build_process -rf" does not quite work yet;
2. revert part of 5969ead9f900 that does not quite work yet: only one accidental host is used;
3. record updates within database, based on serial;
4. proper tables (amending 4611b7b47b42);
5. tuned;
6. clarified signature: improved data integrity;
7. clarified modules;
8. clarified modules;
9. obsolete;
10. misc tuning: prefer Build_Process.Update operations;
11. misc tuning and clarification: prefer explicit type Build_Process.Update;
12. misc tuning and clarification;
13. tuned signature;
14. clarified data representation: more uniform treatment of State.Pending vs. State.Running;
15. tuned: drop pointless SQL.order_by (see also 5f706f7c624b);
16. upgrade pretty_maybe_quote following 2746dfc9ceae;
17. tuned;
18. eliminate odd aliases (see also 2746dfc9ceae);
19. tuned signature: prefer bottom-up construction;
20. tuned;
21. tuned signature;