thread
2. add delete_obsolete_files in doclistmaager, it will find
   status==delting and reference_count==0 file to delete
3. Add reference_count, when adding to kbgroup document reference_count
   will be updated to +1, when deleting from kbgroup it will be updated
   to -1
4. Add KBOperationLogs, currently only deleting file operation will be
   recoreder
5. Modifiy worker thread. when group is default, it will check to delete
   obsolete file

   and  it'll make database abnormal
2. Modify worker's _worker_reparse_files, it will do reparsing without
   change database
3. Move filepath check to DocManager, DocListManager only provides api
   for finding paths in database
4. Modify DocListManager.get_docs_need_reparse, use one query to get all
   docs.
5. Fix lint problem

2. As checking and doing is not done in the same time, add KBPathLock to
   lock the paths.
3. For the same reason, reparsing will not support waiting status.