Simplified some proofs

clarified db content: avoid redundancy of historic ML_IDENTIFIER;

merged

proper filterNot, not filterNot-not;

option build_hostname allows to change hostname easily;

clarified permissions of build.db, following server.db;

enforce rebuild of Isabelle/ML, after various changes to build database management;

misc tuning and clarification: more uniform use of optional "sql" in SQL.Table.delete/select;

tuned: fewer warnings in IntelliJ IDEA;

clarified init_database vs. update_database: implicitly assume fresh "instance";

clarified Build_Process.Context: cover all static information;

tuned whitespace in generated SQL;

tuned: prefer typed operations;

clarified signature: more concise operations;

more robust options in "prefs" format: avoid odd control character;

proper settings for hostname: allow to adjust it in user space;

support for build database: still inactive; more detailed Build_Job.Node_Info;

tuned signature;

clarified signature: more robust operations;

tuned;

tuned;

more operations;

clarified signature: more operations;

clarified signature;