Fixed a race condition in media-monitor

* Prevent exception if files are copied in very quickly
CC-5679: Media Monitor: Uploading folder with tracks will erase organize folder itself

* Fixed problem where the "organize" folder would get deleted if you
  copied a directory into it. Would break FTP uploads on Pro.
Merge branch '2.5.x' of github.com:sourcefabric/Airtime into 2.5.x