FIX: Memory leak in the FileProcessor.

`PendingFile` objects were created to keep track of file chunk uploads. Unfortunately they were not deleted properly afterwards.
This is a nasty bug since the deletion should happen when the `PendingFile` invokes it's callback, telling the `FileProcessor` that it is ready to be harvested and deleted. A deletion in the callback itself caused some nasty race conditions, though. My solution is, to save "completed `PendingFiles`" in a list and delete these list entries from time to time.