discontinued pointless option (reverting 63d55ba90a9f): performance tuning works better via SQL.Database.execute_batch_statement;

clarified database transactions (see also 2c704ae04db1, 7bd0a250183b);

afford multiple tests on fast machines (see also edb4faf666c9 and 2a26d423d9fb); proper history_base for ARM;

performance tuning: parallel and incremental update of build_log_database;

performance tuning: more careful database access;

clarified message;

tuned signature;