Poll for added/altered item directories instead of forcing scan_for_new_items! and update_complete_flag!

[dmolesUC]

This should be feasible even over NFS with listen and the force_polling option.

Note: the option might not even be needed; see listen adapters docs. On the other hand, the polling performance hit isn't really a problem for this use case, and it avoids the risk of running out of inotify watches on a local filesystem, which we easily could if we tried to watch every page scan and OCR file of every item. (Not that we should need to!)