updated properties for scala.concurrent.ExecutionContext.Implicits.global (future task farm), similar to Isabelle/ML; removed obsolete properties for old actors;

simplified change_buffer (again, see 937826d702d5): no thread, just timer, rely on asynchronous commands_changed.post;

replaced manager Actor by Consumer_Thread, which is lazy to defer its start to actual Handler init time;

tuned whitespace;

obsolete;

clarified Session.Consumer, with Session.Outlet managed by dispatcher thread; eliminated old actors;

tuned comments;

more robust prover termination;

more explicit checks;

tuned signature;