add slice_table_test
make unit test passed

- rename rollback to reset_unready
- rename online_fd to file_desc
- fix update(index,.)
- fix unready_after returns wrong range
make online_file compiling passed

- add some logs
- implements Seek
- implements Read
- close all fragment files when close file
- realloc fragment files when resize file
- implement avail
- implement parse_content_range
- refine read_until_n