NFS lock up, timeout needed

@peak3d:
I saw your PR https://github.com/xbmc/xbmc/pull/15686 about NFS timeout.

I have the problem, if my NFS server, from which I tested some video-files, is gone I cannot cleanup my Kodi library. The cleanup progress bar stucks.

Is there a work around? What prevents the implementation of your PR to configure a NFS timeout in the advancedsettings.xml?