Changes

Summary

1. more options for performance tuning;
2. more operations;
3. support for management of build cluster;
4. clarified modules;
5. renamed object Data to private_data, to emphasize its intended scope (but it is publicly accessible in the database);
6. more conservative build_delay (despite 9600720071e6): avoid exessive build_database operations, notably via ssh;
7. proper running limit, based on this worker process; prefer bulk jobs: much faster cancellation;
8. more robust: implicit locking of tables in standard order;
9. more uniform guard (!exists_table(table)): avoid "ALTER TABLE" on already existing table, which could lead to deadlocks if this is presently locked;
10. removed unused "create_index": implicit index from primary_key is usually sufficient;
11. clarified "vacuum" (again, reverting 0bd366fad888);
12. clarified signature: eliminate SQL.Tables.empty to avoid confusion (see also 0bd366fad888);
13. update for release;