Changes

Summary

1. more comments;
2. tuned message -- more readable JSON;
3. more accurate isabelle_id: parent directory is not necessarily at tip;
4. store session: per Server/Context, not Connection; support for "session_stop";
5. clarified server log; tuned options;
6. support for "session_start";
7. updated user name;
8. clarified message;
9. clarified message;
10. more uniform id;
11. prefer non-strict defaults;
12. clarified default;
13. tuned;