Changes

Summary

1. clarified share_common_data: after finished checkpoint, before next edits;
2. obsolete (see 030a6baa5cb2 and d14ddb1df52c);
3. support headless_load_limit for more scalable load process;
4. added dump_options: disabled by default;
5. tuned message;
6. clarified incremental loading: requirements based on maximal nodes;
7. tuned signature;
8. tuned;
9. tuned message;
10. more explicit type Load_State;
11. more operations -- incremental exploration of reachable nodes;
12. tuned messages (again) -- avoid confusion wrt. total remaining size;