Changes

Summary

1. enforce rebuild of Isabelle/ML;
2. proper test (amending 32f9e75c92e9);
3. updated to sqlite-jdbc-3.41.0.0;
4. proper shasum lines (amending 3070001c9d1f);
5. more robust transactions;
6. explicit locking for PostgreSQL --- neither available nor required for SQLite;
7. merged
8. assume total operation: ProcessHandle.current().info.startInstant appears to work on all platforms;
9. more database content, e.g. for monitoring;
10. tuned structure;
11. tuned signature;
12. more database content, e.g. for monitoring;
13. more explicit workers, e.g. for monitoring;
14. tuned;
15. tuned;
16. clarified worker state: always maintain database content via worker_uuid; clarified message;
17. clarified signature: prefer Build_Process.Context for parameters;
18. support for "isabelle build -j0": require external workers to make progress;
19. follow renaming of various Isabelle command-line tools (see b975f5aaf6b8 and before);
20. require the presence of free variables to do abduction in Sledgehammer
21. removed exercise solution
22. merged
23. new theory Tree_Rotations